~shrug~ Totally up to you, but here's my take on it.
If you tear the quilts apart and modify them to your own taste, then they are no longer your grandmother's quilts, and IMO are no longer an heirloom that you've been given custody of. They've become your creation, albeit using the fabric that you got from taking apart a creation of your grandmother's. While the fabric may be vintage, the quilts are no longer antique/vintage.
I don't feel that we can own an heirloom...we're merely given custody of it until it's time for the next caretaker to begin their custodial period. It's like being trusted with a family museum piece.