Phantasmal Fiend

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 target a 1/2 creature then give it +2/+0 later in the turn, it's a 2/3 creature, not a 4/1 creature.

Wotc — March 19, 2021

Because damage remains marked on a creature until the damage is removed as the turn ends, nonlethal damage dealt to a creature may become lethal if you switch its power and toughness during that turn.

Wotc — March 19, 2021

Switching a creature's power and toughness twice (or any even number of times) effectively returns the creature to the power and toughness it had before any switches.

Wotc — March 19, 2021

Effects that switch power and toughness are always applied last when determining a creature's power and toughness, even if other power/toughness-changing effects are created later in the turn. For example, if you activate Phantasmal Fiend's first ability, it will become 2/4. Then if you activate its second ability, it will become 4/2. Then if you activate its first ability again, it will become 3/3.

Wotc — October 01, 2008

Note that the wording "Effects that alter the creature's power alter its toughness instead, and vice versa, this turn" has been removed.

Wotc — November 01, 2005