They do change. Unfortunately, when I met a guy and changed to fit his standards, I lost a lot of friends.

Promising that you won't change seems rather silly, because everyone you spend time with affects you in some way.

Perhaps if you talk to your friend without getting angry or upset and tell her why you're worried about the changes, she will listen to you. She may not change back, at least not immediately, but knowing that you are concerned may help her see how she has changed and think about if it's for the better or not.