Why did HaSh-m tell Yitzchak only now not to go to Egypt - after he had already settled in Eretz Pelishtim?
Ramban #1 (to 26:1): Yitzchak in fact, planned to go down to Egypt, only he went first to Avimelech, with whom his father had made a pact, in the hope that he would offer him refuge, thus sparing him the need to travel all the way to Egypt. Meanwhile, HaSh-m instructed him to scrap his original plans and to remain where he was.
Ramban #2: Pasuk 26:2 precedes 26:1 - as if to say that "HaSh-m had appeared to Yitzchak," before he settled in Eretz Pelishtim.
Malbim (to 26:1): Yitzchak went to Eretz Pelishtim, seeking a place without a famine. When the famine was also there, he intended to go to Egypt.
Ha'amek Davar (to 26:1): Yitzchak trusted that Avimelech, with whom his father had made a pact, would help support him in harsh times. Man's salvation is futile; we find that [after he waxed rich] they hated him and he was loathe to live in the capital city! Therefore, he thought to go to Egypt.
Why did HaSh-m tell him not to go to Egypt?
Rashi: Yitzchak was considered like a Korban Olah (due to the Akeidah), so he could not go to Chutz la'Aretz. 1
Ha'amek Davar (24:6) says that this is why Avraham was adamant that Yitzchak not go to Charan to marry. Did Avraham not tell him that he may not leave Eretz Yisrael?! (PF)
QUESTIONS ON RASHI
Rashi writes: "Do not descend to Egypt - for Yitzchak had intended to descend." How do we know this?
Gur Aryeh: Otherwise, why would HaSh-m need to warn him not to do so?
Rashi writes: "Yitzchak planned to go down to Egypt." What was Yitzchak's reason?
Ramban: Refer to 26:1:1:2.
Seforno: Because there was not sufficient grass for his flocks to graze, and he assumed that he would be better served in Egypt.
Rashi writes: "He had intended to descend, as his father (Avraham) also had, in times of famine." Why must we explain that Yitzchak was tracing his father's footsteps? Perhaps he made the decision to descend on his own!
Gur Aryeh: The verse compares this famine to the one in Avraham's time, because that was Yitzchak's basis for his decision. Although such a harsh famine is sufficient Halachic justification to leave Eretz Yisrael (Bava Kama 60b), Yitzchak did not decide to leave on his own, for HaSh-m had already blessed him (25:11), and he would surely have what to eat. But Avraham had in fact left Eretz Yisrael due to a famine, even after HaSh-m blessed him (12: 1-3). Yitzchak would have followed suit, 1 had HaSh-m not commanded otherwise. HaSh-m blessed Yitzchak again now (26:3), during the famine, and as such there was no longer cause for leaving.
Gur Aryeh: This implies that HaSh-m's blessing of Avraham's property only applied to normal times, and not to a famine; and Yitzchak assumed that his blessing would be likewise.