I'm sending my players to a demiplane of madness inspired by Xoriat. Tips?

[Riverdance]

I... really like this, actually. Should I make them trade character sheets or just apply the physical changes?

The monsters thing sounds cool too, but it would be really hard to pull off. Same with the shifting maze.

I'd have them switch character sheets but keep their old weapon proficiencies and skills, etc... Say the Wizard and Fighter switched. Suddenly you would have a hulking meat-shield who has almost no melee experience, and frail old guy in robes who knows how to wield a greataxe (even if he might not be strong enough to).

I think the best way to do this would be to do this would be to have them switch character sheets, but carry over mental scores, as well as skills and feats from their original characters (you could nerf/boost charisma slightly if a character is particularly ugly/attractive but honestly that isn't a large part of most charisma based checks). That way, the rogue who is now in the cleric's body still knows how to pick locks (same intelligence and skills), but it may be harder now that he's in a different body (lower Dex).

Make sure you check that it makes sense for all the skills and feats to carry over first. For example, Toughness would not go with the fighter when he transferred into the wizard's body.

EDIT: Now that I've worked out how to do this I really want to try it sometime.