I don't think you can just cut the piece like that. If you wanted to reshape the hairline the best bet is to remove all the hairs until you get to the new outlined hairline. I know salons do this but it can be costly. Removing individual hairs is tedious. I remember reading about someone actually taking nair hair remover to the underside of a piece and having the hair come right off. So that might be a quick option. But once you get to your new hairline like hairenough said... the "new hairline" would need to be graduated with proper density and all the knots invisible etc.