You cannot usually switch from Anytime to Traditional because Traditional is usually waitlisted. You can, however, easily switch from Traditional to Anytime (at least you could on my recent Sapphire cruise). Traditional diners were allowed to use the Anytime dining rooms on a walkin basis (they could not make reservations) any night they wanted without losing their seat in Traditional. If they wanted to be able to make reservations, then they had to give up their seat in Traditional so that someone on the waitlist could have it.
Anyway, that is how it was SUPPOSED to work. In reality, we met several people who had Traditional dining who WERE ABLE to make reservations for Anytime without any problems. I don't think they keep very good track of who is making reservations.