Or you do it the same way everyone else does it when the rules change: everyone who already has Banned Thing is grandfathered in but it's no longer transferrabl

Or you do it the same way everyone else does it when the rules change: everyone who already has Banned Thing is grandfathered in but it's no longer transferrable or acquirable. Or you do it the Valve way: give everyone the Thing and make existing Things the Extra Special Limited Edition Thing that's functionally identical but still confers bragging rights - this would still be within Mojang's rules.