If you only sail local events the number is fine. If you want to sail Region, Masters, North American events, then you would need to join the Class. If you can have Gregg release the sail number to you, it can be reassigned. Otherwise, the Class will assign a new number.
Most regatta hosts overlook incorrect sail numbers. (Officially not supposed to but ...) If Gregg showed up with another boat with "his" number, though, you'd have to change. If you don't get his number released to you, when you get a new sail, then put on your "Class" sail number.