I'm sorry to hear that you have to leave the store you are so happy at. The manager must really feel like you are up to the task though, or he wouldn't have asked you to do it.

I know how you feel about having to change, though- at every new job I'm at, I am terrified the first day...I know my fears are groundless, but that doesn't stop me from feeling them! Gradually, you will get used to working in the new store- maybe you will even like it better! And you will have your mom with you. Good luck, I hope things turn out well.