Gemara

65b (R. Ila'i): If one stole a lamb or calf, and it grew up and became a ram or ox, this is a Shinuy (change), and he is Koneh (acquires it, but he owes its value). If he slaughters or sells it, he slaughters or sells his own animal.

66b (Mishnah): Hides of a Ganav (covert thief) can receive Tum'ah through intent (to use it as is, for a table-cover). Hides of a Gazlan (open robber) cannot.

R. Shimon says, hides of a Ganav cannot receive Tum'ah through intent. Hides of a Gazlan can, because the owner despaired.

Inference (Abaye): This shows that one acquires through despair of the owner!

Rebuttal (Rava): A change in the name of the object is like a physical change.

One acquires through physical change, e.g. he stole wood, and now it is Kelim. Similarly, it was called a hide, and now it is called a table-cover.

Question: Regarding a beam, Shinuy ha'Shem (to 'roof') does not acquire!

(Mishnah): If one built a stolen beam into his house, he returns its value. This is an enactment to help thieves repent. (Letter of the law, he did not acquire!)

Answer (Rav Yosef): It is still called a beam, even after it is built into the roof.

68a (Beraisa): If Reuven stole an animal and slaughtered it, then Shimon stole it, Reuven pays four and five, and Shimon pays only principal.

Objection (Rava): All agree that Shinuy Koneh. Shimon owes Kefel (to Reuven)!

Answer #1(Rava): The text should say that Shimon pays Kefel.

Answer #2 (Rav Papa): The Beraisa is Beis Shamai, who say that Ein Shinuy Koneh (it does not acquire).

93b (Mishnah): If one stole wood and he made Kelim, he pays the initial value.

Inference: He pays the value at the time he stole it because he made Kelim. Had he only smoothed the wood, he would not!

95a (Beraisa - R. Meir): If one stole a sheep and sheared it, or a cow and it gave birth, he pays for it and its shearings and children;

Version #1 - Question: Does R. Meir hold that Shinuy does not acquire? Or, does he hold that Shinuy Koneh, but here we fine the thief, lest he profit from his sin?

Answer (Mishnah - R. Meir): If Reuven gave wool to Shimon to dye red, and he dyed it black, or vice-versa, Shimon pays him the value of the wool he received.

Inference: He does not pay the added value of dyed wool. If he held that Ein Shinuy Koneh, he should pay for dyed wool! Rather, he holds that Shinuy Koneh.

Version #2: R. Meir holds that Shinuy Koneh, for Rav switched the Mishnah (about a stolen animal that grew old) so that R. Meir says that he pays like the time of the theft.

Rishonim

Rif and Rosh (27a and 7:2): If one stole a lamb or calf and it became a ram or ox, he pays Kefel like he stole, i.e. lambs. If he slaughtered or sold it, he is exempt from four or five, for he acquired it through Shinuy, like R. Ila'a taught, so he slaughtered or sold his own animal.

Rambam (Hilchos Geneivah 1:12): If the stolen object changed in the thief's hands, he acquires and its improvements, even before despair. He pays only its value.

Rambam (14): If one stole a lamb or calf, and it grew up and became a ram or ox, this is a Shinuy. He acquires it and pays like at the time of the theft, even if the owner did not despair.

Rosh (9:1): The Mishnah says 'and he made them Kelim.' I.e. merely smoothing the wood does not suffice unless it is called a Kli or the name changed.

Poskim

Shulchan Aruch (CM 353:1): If the stolen object changed in the thief's hand, e.g. he stole a lamb or calf, and it grew up and became a ram or ox, he acquires it through Shinuy ha'Shem.

Rebuttal (Yam Shel Shlomo 7:5): (This is like the Rosh.) What is his source to say that a lamb that became a ram is a proper Shinuy? Perhaps he learns from the Stam Gemara, which asked that he should acquire due to Shinuy ha'Shem. Tosfos (67a DH Ha) says similarly. However, Rava, who is Basra, proved from verse that this is not a Shinuy ha'Shem! Rather, R. Ila'a and R. Chanina argue about Shinuy Ma'aseh. A small animal that became big is the ultimate Shinuy Ma'aseh. R. Chanina holds that a Shinuy that comes by itself does not acquire. Tosfos (68a DH Mah) says that other Amora'im disagree. Our Gemara asked that all should agree that Shinuy ha'Shem acquires by itself; Rava answered that there is no Shinuy ha'Shem here.

Suggestion: Even though it was initially called a ram, it is still Shinuy ha'Shem because it used to be called (also) a lamb, and after one year it is not!

Rebuttal (Yam Shel Shlomo): A beam that was built into the roof is not a Shinuy ha'Shem, even though it is now called roof, for it is still called (also) beam. Even though there it gained a new name, and here it lost a name, it is unreasonable to distinguish between these. Perush ha'Mishnayos (9:1) says that he acquires because it became big. I.e. now it is big in years (old). The primary Shinuy ha'Shem of maturing is that it is no longer called a calf, rather, an ox. Rava did not need to say why it does not acquire through Shinuy ha'Shem from small to big. Indeed, the Gemara's question proves that Shinuy ha'Shem acquires, but all agree with Rava that becoming a ram or ox is not Shinuy ha'Shem. According to the Rosh, if one stole a lamb exactly one year old, he would acquire in one day, for the next day it is called Palgas (it is not Kosher for Korbanos that require a lamb). Or, if he stole a Palgas exactly 13 months old, he would acquire in one day, for the next day it is called a ram, even if it did not grow. I hold that growth during the first year or during the second year does not acquire, for it is not Shinuy Ma'aseh. We do not find that one who stole a ram or ox acquires if it gets bigger. This would be a bigger Chidush! Rather, we require physical growth and a change in name.

Defense #1 (Shach 1): We follow the way people speak, even to be lenient. We see that people do not call a newborn a ram or ox. It seems that the same applied the days of the Gemara. If not, R. Zeira had no question! Rava answered that since the Torah calls them ram and ox, we can say that people did so the days of R. Ila'a. We say like this in Nedarim (49a).

Defense #2 (Gra 1): The Rosh merely gave an example of Shinuy ha'Shem, i.e. according to R. Zeira's Hava Amina. Really, an animal that matures is a Shinuy Ma'aseh.

Rema (360:6): Something is called a Shinuy (Ma'aseh) only if the name of the stolen object changed due to the Shinuy.

Gra (10): There are exceptions. The Gemara suggested that the myrtle vendor should acquire through Shinuy Ma'aseh, and afterwards it suggested that he should acquire through Shinuy ha'Shem, and said that there was no Shinuy ha'Shem. A thief acquires a lamb that matures, even though there is no Shinuy ha'Shem.