Oh no!! I did something similar, one side of my backing kidn of curved in on one side and I thought the back would be big enough going the other way that it wouldn't be a consideration. We ll I was wrong and I didn't know it till I was almost done basting too. So now one of my quilts has a curved edge LOL, I was more mentally capable of coprehending cutting my quilt top to match than unpinning and repinning the whole thing. Oh well, it is a much loved home use quilt anyway.
Good luck!!
Rachel