Found the answer I case anybody else is wondering how to change costumes:
"To change costumes, the player must complete Jill or Chris' story modes with the best endings. After the ending, a screen will display showing that the Closet Key is unlocked. The key unlocks the closet door in the costume room. Inside, Jill and Chris can choose between costumes."
You say replaces tank top, but how to I change Jill's outfit and/or at what point in the game does she wear a tank top ? At the beginning of the game she is not wearing a tank top and Jill is not using your American Girl outfit sadly.
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"To change costumes, the player must complete Jill or Chris' story modes with the best endings. After the ending, a screen will display showing that the Closet Key is unlocked. The key unlocks the closet door in the costume room. Inside, Jill and Chris can choose between costumes."