Should I forget about it or not?

Go, have a good time, if you end up asking her out then that cool, and you need to just live life and live in the moment as cheesy as it may sound. You don't want to look back and wonder what if she would have stayed, or what if I would have asked her out, what if I would have just gone to prom with her? It might not work and she might move, but people do the long distance relationship every day and heck she might not go you will never know what will happen unless you just give it a shot.

You never know unless you do it, take it one day at a time and do what makes you happy - if you like her and want to go to prom do it, if later you feel hesitant about dating or feel stronger about asking her out go with that feeling as well. It is petter to live your life and no look back at the what ifs that way you can look back and know it played out how it could have. Hope I helped at least a little good luck :)