Why can't I change a full member to a Guest?

@Sarah Bowman - Because Guests are only added directly to a board on a board-by-board basis, if there was an option to demote a full Member to a Guest, Miro would not know which board(s) you would want to add them to, and it would not make sense in the context of changing a user role to then also ask you which board(s) they should be a Guest of. And, even though they were initially added to at least one board before they were promoted from Guest to full Member, once that action occurred, they were no longer a Guests on that (or any board). I hope that makes sense why there is not an option to change them back to a Guest :)