Is it permissible to change the name of a newborn girl, "Khadijah" - a name that was vowed for her and with which she was blessed - to another modern name, due to the mother's feeling that the name is burdensome and her fear that her daughter will not accept it in the future?

This vow is of the type that concerns a permissible act. The preponderant opinion is that it is not binding. However, since you have fulfilled it and named the girl Khadijah, that is the most cautious approach, to avoid disagreement. Changing the name is permissible if the other name is free from any Shariah prohibitions, while reminding that the name Khadijah is one of the best Islamic names for girls.