Gemara

101b Rav Chisda: Shinuy Makom obligates a new Berachah only if one ate foods which do not require a Berachah (Acharonah) where they were eaten;

Rav Sheshes: This applies to any food.

Question (against Rav Chisda - Beraisa): If friends were reclining to drink and uprooted themselves to greet a groom or bride, they need not bless before leaving or when returning;

This is when an old or sick person remained there - if not, they require a Berachah Acharonah on what they ate, and a new Berachah Rishonah when they return.

'Uprooted' connotes that (normally, they would not leave suddenly without blessing, i.e.) they were eating foods that require a Berachah in their place - nevertheless, they must bless before leaving and when returning unless one of them remained!

Answer (Rav Nachman bar Yitzchak): R. Yehudah taught the above Beraisa (Rav Chisda holds like the first Tana of the coming Beraisa. Whenever R. Yehudah says 'when is this?', he argues with the previous Tana):

(Beraisa): If friends were reclining and uprooted themselves to go to synagogue or the Beis Medrash, they need not bless before leaving or when returning;

R. Yehudah: When is this? This is when some of them remained there - if not, they require a Berachah Acharonah on what they ate, and a new Berachah Rishonah when they return.

Inference: This is because they were eating foods that require a Berachah in their place - but regarding foods that do not require a Berachah in their place, even Chachamim mandate blessing before leaving and when returning!

Support (for Rav Chisda - Beraisa): If friends were reclining to drink wine and uprooted themselves and returned, they need not bless (Rashi - it specifies wine because it requires a Berachah in its place).

Chulin 107b - Rav: A waiter blesses before every cup of wine he receives, but not on every slice of bread.

R. Yochanan: He blesses before every slice of bread.

Rav Papa: They do not argue. He need not bless for every slice only when an important person is present (then he knows that he will get as much bread as he needs).

Rishonim

Rif: The Halachah follows Rav Sheshes.

Source: A Beraisa challenges Rav Chisda, and a poor answer was given.

Rebuttal (Rosh 10:6): The Rif's proof is not solid, for the Beraisa against Rav Chisda is like R. Yehudah, and Chachamim argue with him! Also, a Beraisa supports Rav Chisda.

Rebuttal of Rosh (Kesef Mishneh, Hilchos Berachos 4:4): Indeed, Rav Nachman bar Yitzchak holds that the Beraisa against Rav Chisda is like R. Yehudah, but Chachamim argue. However, we hold that 'ba'Meh Devarim Amurim' comes to explain, i.e. Chachamim do not argue! The Gemara did not challenge Rav Sheshes from the Beraisa supporting Rav Chisda; this shows that the Beraisa is an individual's opinion.

Rambam (Hilchos Berachos 4:3,4): If people were eating and left, leaving no one behind, when they return they must bless retroactively on what they ate and say a Berachah Rishonah before eating more. The same applies if they were drinking or eating produce.

Kesef Mishneh: The latter clause in the Rambam shows that the first clause discusses foods that require a Berachah Acharonah where they were eaten. This is like Rav Sheshes.

(Rosh 10:6): The Halachah follows Rav Chisda. Shinuy Makom is like Hesech ha'Da'as (taking one's mind off eating), therefore it obligates a new Berachah Rishonah. However, one Berachah Acharonah suffices for what was eaten in both places. If one ate food that requires a Berachah Acharonah in its place, and went elsewhere, since if he will not eat here he must return to his original place, it is as if he is still there. He may finish eating and make the Berachah Acharonah here.

Beis Yosef (OC 178 DH v'Chosav ha'Ran): Hesech ha'Da'as does not obligate a new Berachah Acharonah, for there is no concept of interruption between the action (eating) and a Berachah Acharonah.

Source - Gra (178:2 DH v'Yesh Cholkim): We learn from Chulin 107b. A waiter blesses on every cup because he did not expect to get more, but he need not bless for what he already ate. Here, Tosfos (DH keshe'Hen) proves from the Beraisa that the same applies to Shinuy Makom. If it were necessary to bless retroactively on what was eaten, the Beraisa would not need to say that one needs a new Berachah Rishonah to eat more!

Beis Yosef (ibid.): The Beraisa says that they bless on what they ate before. The Rosh holds that this is recommended before leaving, lest they delay so long that a Berachah Acharonah will not apply to what they ate, i.e. after the food is digested.

Beis Yosef (DH u'Mashma): Those who Poskin like Rav Chisda do not distinguish whether or not he left people eating. In either case Shinuy Makom obligates a new Berachah only on food that does not require a Berachah in its place. Even if he ate with others, people are not Kavu'a (set) to eat such things together.

Poskim

Shulchan Aruch (OC 178:2): If people were eating and left, leaving no one behind, when they return they must bless retroactively on what they ate and say a new Berachah before eating more.

Mishnah Berurah (19): Hesech ha'Da'as obligates a new Berachah Rishonah but not a Berachah Acharonah on what was already eaten. Shinuy Makom is different, for what one eats afterwards is a totally different meal.

Shulchan Aruch (ibid.): The same applies if they were drinking or eating produce, for a Shinuy Makom ends a meal. Therefore, he must bless on what he ate and before eating more.

Gra (DH v'Davka): Even though the Beis Yosef said that Kevi'us does not help for food that does not require a Berachah in its place (above b:3:iv), he says differently in the Shulchan Aruch. 'The same applies...' connotes that Kevi'us helps also for such food, like the Rema below (4).

Note: The Rama requires Kevi'us only for such food.

Source (Gra DH v'Davka): The Gemara learns from 'uprooted' that the Beraisa discusses food that requires a Berachah in its place. If Kevi'us helped only for such food, the law itself (that leaving people behind precludes the need for a new Berachah) would prove that it discusses such food! Rather, we must say that Kevi'us helps for all foods.

Taz (3): The Shulchan Aruch is consistent with what he wrote in Beis Yosef. 'The same applies...' does not connote exactly the same law, for Kevi'us helps only in the first clause.

Note: Almost all Poskim explain like the Gra, unlike the Taz.

Shulchan Aruch (ibid.): One who changes his place within a house need not go back and bless. One who changes from one side of a tree to the other must bless again.

Taz (4): If he ate in the second place, he could bless there! Rather, the Shulchan Aruch discusses one who returned to his initial place to finish his meal, and whether or not he must bless retroactively on what he ate and say a new Berachah Rishonah before resuming.

Rema: Some say that Shinuy Makom is only like Hesech ha'Da'as, so he need not bless on what he ate, only on what he intends to eat now. If he was eating with people and some remained, or if he ate things which require a Berachah where they were eaten, he need not bless again.

Magen Avraham (7): One who was eating (food that does not require a Berachah in its place) with people is exempt from a new Berachah only if he returns to them. We learn from Rav and Shmuel, who require people to say Borei Peri ha'Gafen at home, even though they started drinking with visitors who remained (eating) in the Beis ha'Keneses.

Rema: All agree that one must bless on bread where it was eaten. Therefore, (in any case) if one was eating bread, he need not bless again. However, if he had Hesech ha'Da'as, he needs to bless before eating more. We do not distinguish whether he finishes his meal in the original place or elsewhere. L'Chatchilah, one may not leave his place without blessing first, lest he forget to return and bless. For the sake of a Mitzvah that cannot be done later, such as prayer, this is permitted.

Magen Avraham (8): If one was invited to eat elsewhere, surely he will not forget to bless (he will bless there), so it should be permitted to leave without blessing! However, the Ran, the Bach and the Rama himself forbid if at the time of ha'Motzi he did not intend to finish eating elsewhere.

Mishnah Berurah (28): Even regarding bread, one should not leave his place before eating a k'Zayis. Less than this does not require a Berachah Acharonah, so it is like a food that does not require a Berachah in its place.

Kaf ha'Chayim (14): Here, even we (who normally follow the Mechaber) follow the Rama, because we are lenient not to say Safek Berachos.

Kaf ha'Chayim (15): If one leaves his place and continues to eat while he goes, this is not a Shinuy Makom.

Gemara

R. Yehudah: If one was eating when Shabbos came, he makes Kidush on one cup of wine and says Birkas ha'Mazon on a second;

R. Yosi: He finishes eating, even if it is after dark. Afterwards, he says Birkas ha'Mazon on one cup and Kidush on another.

Rishonim

Rif: Above (100A), we concluded that the Halachah is not like R. Yehudah or R. Yosi! Rather, one spreads a cloth and says Kidush, like Shmuel. Nevertheless, we learn from our Gemara about people who finished their meal and washed Mayim Acharonim, but did not say Birkas ha'Mazon until Shabbos came. They say Birkas ha'Mazon on one cup and Kidush on another, like R. Yosi.

Ran (DH Kos): We learn from R. Yosi that even though one will not eat after Kidush, Kidush is considered b'Makom Seudah because he finished his meal there.

Machatzis ha'Shekel (OC 274:2): Even if it is considered b'Makom Seudah, surely it does not count towards the three meals one must eat on Shabbos.

Rif: We may not say both of them on one cup, for they are different Kedushos (Mitzvos). Kidush and Havdalah may be said on one cup, for they are essentially the same Mitzvah.

Rosh (10:8): This is because Havdalah from Shabbos to Yom Tov also mentions Yom Tov.

Rambam (Hilchos Shabbos 29:13): They say Birkas ha'Mazon on one cup and Kidush on another. We may not say both of them on one cup, for they are Mitzvos mid'Oraisa.

Rosh (10:7): A Tosefta requires mentioning Shabbos in Birkas ha'Mazon if one ate also after dark. Presumably, the same applies when he did not eat after dark, just like one says an extra Tefilah of Shabbos to compensate for missing Minchah on Erev Shabbos. The Halachah follows Shmuel, who requires spreading a cloth and saying Kidush before continuing to eat, therefore one may not say Birkas ha'Mazon over a cup (and drink it) before Kidush. Rather, he must make Kidush before Birkas ha'Mazon.

Rosh (10:8): Because we do not say two Kedushos on one cup, the six Berachos (of Sheva Berachos, excluding Borei Peri ha'Gafen) should not be on the same cup as Birkas ha'Mazon. R. Meshulam holds that we say the Berachos of Kidushin and Nisu'in on the same cup. R. Tam holds that we say them on different cups, for sometimes Nisu'in is much later. Also, (even when Nisuch is right after Kidushin) in many places the Kesuvah is read in between.

Poskim

Shulchan Aruch (OC 271:6): He says Birkas ha'Mazon on one cup, and Kidush on a second.

Kaf ha'Chayim (41): If there are less than three people, they bless Birkas ha'Mazon without a cup.

Shulchan Aruch (ibid.): He mentions Shabbos in Birkas ha'Mazon, even though this is before Kidush.

Magen Avraham (14): Even though it is doubtful whether or not one must mention Shabbos in Birkas ha'Mazon, it does not hurt to mention it.

Rema: Some say that he does not mention Shabbos in Birkas ha'Mazon, for this depends on the beginning of the meal. Opinions vary concerning whether or not he tastes from the cup of Birkas ha'Mazon before Kidush, and whether or not he must eat after Kidush so that the Kidush will be b'Makom Seudah.

Gra (DH v'Yesh): The opinion that permits tasting the wine considers the cup of Birkas ha'Mazon to be part of the meal. The opinion that requires eating after Kidush explains that R. Yosi holds like Rav, who does not require Kidush b'Makom Seudah, but we hold like Shmuel.

Kaf ha'Chayim (47): The opinion that requires eating requires a k'Zayis to be considered b'Makom Seudah. It is better to eat a k'Beitzah, for this is a more substantial meal to honor Shabbos.

Shulchan Aruch (ibid.): Some say that even if he finished his meal he does not bless Birkas ha'Mazon now. Rather, he spreads a cloth, makes Kidush, blesses ha'Motzi, eats a bit, and then says Birkas ha'Mazon.

Magen Avraham (18,13): He says ha'Motzi if he already washed Mayim Acharonim; he must also bless Borei Peri ha'Gafen. If he did not yet wash, all agree that he spreads a cloth and says Kidush.

Rema: The custom follows this opinion to avoid the dispute about the first opinion.

Mishnah Berurah (36): The latter opinion does not fulfill the first opinion! Nevertheless it is better, for the first opinion requires saying Birkas ha'Mazon, and according to the latter opinion these are Berachos l'Vatalah.