Reverse the Polarity

Effects that switch a creature's power and toughness apply after all other effects, regardless of when those effects began to apply. For instance, if you switch a 2/4 creature's power and toughness and then give it +2/+0 later in the turn, it's a 4/4 creature, not a 6/2 creature.

Because damage remains marked on a creature until the cleanup step or an effect removes that damage, nonlethal damage dealt to a creature may become lethal if you switch its power and toughness during that turn.