There is no premium version yet. It will come later, but I think it should be retroactive and all 10+ support should give access to the premium the day it's available
As far as I know, as long as you collect pay what you want, anyone paying anything will have status of a buyer and if you set a minimum price later, that status won't change.
But if you set price tiers for files that is probably different, as it should compare the tier with the paid amount. The only stupid thing is, that one cannot increase the paid amount.