Why from the three Avos, Avraham and Yaakov had their name changed, and not Yitzchok?
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Yitzchak's name was bestowed by Hashem (Gen. 17:19).
Actually, Rashi (first explanation to Gen. 25:26) says that Yaakov was also named by Hashem. Yefeh Mar'eh simply says that the Yerushalmi evidently agrees with Rashi's second explanation, that he was named by Yitzchak. Tov Ayin, on the other hand, suggests that the difference is that Yitzchak was named before his birth, and Yaakov only afterwards.
The sefer גבעת שאול here explains that there are three types of names that Hashem gives:
The first is a name which Hashem gives something/someone that does not previously have a name, and this is only name that the thing/person has. The second is when Hashem changes a person's name slightly, by adding a letter or by a slight change in the letters, but essentially retaining the original name. The third is when Hashem gives someone an additional name and retains the person's original name as well.
And a perfect example of these three types is the names of the Avos: Yitzchok was of the first type, because this was the name given to him by Hashem before he had any other name, as it says "but Sarah your wife will give birth to you a son, and you will call him Yitzchok". And Avraham was of the second type, because his first name was Avram which was given to him by his father, and Hashem changed it by adding the letter 'heh' to make his name Avraham, but his original name essentially remained. Finally, Yaakov was of the third type, because Hashem left his first name in place and gave him a new, additional name.
Now, it is well known that the name of a thing/person describes some quality of that person, and it seems to me the effect of these different types of names that Hashem gives is that the greater the change of the name, the greater the quality described by that name is increased. Therefore, when necessity dictates that a name be given to someone to distinguish him from someone else, he will have the quality described by the name, but in a relatively small amount. Therefore, the name Yitzchok teaches about the miracle that occurred with his birth, that "everyone who hears will laugh", and the name Yitzchok gave existence to that miracle. Similarly, Yishmael whose name was mentioned before he was born teaches that Hashem would listen to his voice when he was in trouble, as we see happened. The name Shlomo teaches that there would be peace in his days, and the name Yoshiyohu (יאשיהו) teaches about his zealousness for Hashem that would be in his heart (אש י-ה) to remove all the idolatry from the land.
But those who already had a name and subsequently Hashem changed it slightly, this teaches that the effect which their name signifies is much greater than the first type, and will spread across time and affect other people. For example, the name Avram signified that he was the father only of Aram, but Avraham signifies that "I will make you the father of a multitude of nations". Sarai signified that she had kingship over only a few, but the name Sarah signifies that this effect would be over the whole world, that from her would come forth kings. And the name Yehoshua taught about the great salvation that there would be for Yisrael through the conquering of Eretz Yisrael.
However, the third type where the change was even greater with the addition of a new name, this teaches that the effect of that name is far greater and more complete than all of them, because the new name signifies the eternity of the person who, before the new name was given to him, would have died. Therefore, by changing Yaakov's name to Yisrael He made him eternal, and also gave him the effect that the name Yisrael signifies. [This also gives an understanding of the teaching of Chazal: יעקב אבינו לא מת - my comment]