Op, this is very common

I went by a nickname all through school - including college. Then I switched to my middle name before grad school and have used that name ever since. It took me about 2 weeks to get used to it because I started it when I arrived at graduate school. The only issue I have is when I go home, some of the "family" still call me by my nickname - although their kids all call me aunt "middle name". Not quite the same as changing it when your in elementary school, but know its done all the time.

Girl I went to school with started going by her middle name in about 3rd grade. That was 40 years ago and she still goes by her middle name. DH's best friend changed his name in his late 20s. It took a little while to get used to it but I don't even think about his old name any more. Let her try it for a while.

My DD did that starting in first grade, because she didn't want to be the fourth girl named Isabella. So she announced she was going by her middle name instead. I think it took people about a week or two to get used to it and then it was fine. We've been calling her by her middle name for years now.