Note that the wording "Effects that alter the creature's power alter its toughness instead, and vice versa, this turn" has been removed.
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.
Effects that switch power and toughness apply after all other effects that change power and/or toughness, regardless of which effect was created first.
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.