I think here (as always), honesty is the best policy. Since applying, you have realized your REA school is “not the best fit for” you. I’d suggest having a sincere conversation with GC expressing why you now think it’s not a good fit, and how your #2 choice suits you much better. If you have remotely reasonable rationale, they have to be pretty awful to not feel sympathy for your situation. It’s possible this will be effective. So I’d put my energy into outlining the fit issues. Good luck! (But yes, as mentioned above, I would not decline your REA until you have another preferred acceptance in hand).