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.