Make the relevant individual cards available through BoardGameGeek, just like promos. Start by trying just one card (e.g. Band of Misfits) and if that proved there was sufficient demand, gradually work through the others, doing the ones with the most significant changes first.
The promos are given to BGG, to support BGG; RGG just loses money on them. So, what you're saying is, instead of selling these cards, we could give them away. That's certainly technically accurate, it's another option we have not chosen.
Make all individual cards available postally from RGG. That would also be a boon for anyone who has been cursed by damaging a few cards in a set, e.g. by spilling a drink over them, and wishes to replace them because currently they don't have a ghost of a chance of not recognising the damaged cards when shuffling their deck.
This also incurs real costs.
Another thing we could do is, we could never change wordings on cards, just live with all mistakes, because there were people out there who wanted to be the reason we can't have nice things. We decided against that, we fixed some wordings and are not providing them separately. Take that, those people.