I know that, according to this FAQ
, moves cannot be undone, even if both players are willing.
But what about a means by which the time controls for a game can be changed, if both players are willing? In other words, if I start a game with Person A with 7/7 time controls, and we both agree to change to 1/7 time controls half-way through, it seems reasonable enough to allow for this to happen.
I can see the need for exceptions to this rule for "special" games--tournaments, ladders, sieges, etc.--but there wouldn't seem to be a problem (to me at least) to allow this for "normal" games.