Your name shall no longer be "Avram"; it shall now be "Avraham," because I have made you a father (Av) to a multitude (Hamon) of nations.

(Bereishit 17:5)

One who calls Avraham by the name "Avram," transgresses a positive... and a negative command, as the verse says, "Your name shall no longer be Avram."

(Berachos 13a)

The Mechilta (Parshat Bo #16; as cited by Rav Yakov ha'Gozer in his rules of Milah, p. 96) suggests a straightforward explanation: Yakov was given an entirely different name from his previous name. His new name may therefore be classified as an *additional* name. Since he now has two names, it is permitted to use either one of the two, Yakov or Yisrael; they are both his names. Avraham, on the other hand, was not given a new name. Rather, his old name was *corrected* by the addition of the Hebrew letter "Heh" in middle of the name. Since his original name is now considered incorrect, it is no longer appropriate to use it. This is why the name Avraham prevails.

(1) The Yerushalmi (Berachos, end of ch. 1, cited by Rav Nisin Gaon Berachos 13a) addresses the matter of why Yitzchak' name was not changed at all. Avraham's original name, points out the Yerushalmi, was given to him by his father, Terach. Yakov's name was given to him by his father, Yitzchak. Yitzchak's name, though, was given to him directly by G-d Himself (Bereishis 17:19). Therefore, there was no need to change it!

(It should be noted that Rashi Bereisit 25:26 cites one opinion who maintains that it was indeed Hashem who gave Yakov his name, and not Yitzchak. However, this does not mean that Hashem appeared to Yitzchak and commanded him to call his son by the name Yakov (as Hashem appeared to Avraham and commanded him to call his son Yitzchak). Rather, it means that when it came time for Yitzchak to give his son a name, Hashem "put the name" of Yakov into Yitzchak's mouth through divine inspiration.)

Along the Yerushalmi's line of reasoning, the Mechilta (ibid.) fills in why Yakov's name was not changed permanently (but rather an additional name was given to him) and Avraham's was. Avraham's original name had to be entirely discontinued since his original name was given to him by his father Terach, who was an idolater. Yakov's original name could continue to be used, though, since his name was given to him by his righteous father Yitzchak. And Yitzchak's name, as we have seen, did not have to be changed at all, since Hashem Himself administered it.

It may be added that this approach may also explain why Avraham and Yakov's names were changed the way they were changed. Avraham, who was the first to leave the idolatrous ways of the earlier generations, did not follow his father's ways in anything that he did. According to the reasoning of Rabbeinu Chananel, it was therefore appropriate for Avraham's name to be changed entirely. Yakov, on the other hand, followed in the ways of his fathers. However, in one instance he caused his father's will to be breached. Yakov's father called his son Esav. Yakov, on the other hand, caused Esav to be called "Edom" (Bereishit 25:30). Because Yakov brought about this name-change against the will of his father, his name was also changed. Since in general he did follow his father's ways, his name was not entirely. He was given a new name, but the old one still remained, And as we have seen, since Yitzchak did not veer from his father's will, and he did not change the names his father gave to the wells in even the slightest fashion, his name was not changed at all!