I don't see how that would be a problem! A retcon/change will probably stay at two tokens because going forward that also encourages people to think it through what powers they want (but doesn't leave them stuck with ones they don't want indefinitely).
edit: older players in your case I think might get a retcon for free now that I'm thinking about it, since as you said, shapeshifting becoming a freebie invalidates one of those powers (unless you wanna do something different with the forme ability, since shapeshifting doesn't grant special abilities that may be associated with a specific form otherwise)
no subject
edit: older players in your case I think might get a retcon for free now that I'm thinking about it, since as you said, shapeshifting becoming a freebie invalidates one of those powers (unless you wanna do something different with the forme ability, since shapeshifting doesn't grant special abilities that may be associated with a specific form otherwise)