Its specific wording there, teaches us some of the mitzva's details. And if you will say that the first time the mitzva is mentioned is from where it is logical to count it, know that this is not the case with several mitzvot. And specifically, when a mitzva is mentioned in the Torah before Matan Torah, and again after Matan Torah, it is usually counted from the latter source. This can be seen by looking at the brachot that are said for the mitzva. The context of Lech L'cha says it all.
Name changes represent changes in character, role, and destiny.
G-d promises that Sarah will bear the true heir of Avraham. Avraham laughs with joy upon hearing that he will be a father at 100, and Sarah a mother at 90. Yishmael will also be blessed and give rise to great nations, but the Covenant will be passed down through Yitzchak. Avraham circumcises himself at age 99, Yishmael at age 13, and the other male members of his household, in fulfillment of G-d's command. The final four p'sukim are repeated for the Maftir.