2005-11-01 : Note that the wording “Effects that alter the creature’s power alter its toughness instead, and vice versa, this turn” has been removed.

2008-10-01 : 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.

2013-04-15 : Effects that switch power and toughness apply after all other effects that change power and/or toughness, regardless of which effect was created first.

2013-04-15 : 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.