The effects from Kellan's abilities overwrite other effects that set power and/or toughness if and only if those effects existed before the ability resolved. They will not overwrite effects that modify power or toughness without setting it (whether from a static ability, counters, or a resolved spell or ability), nor will they overwrite effects that set power and toughness which come into existence after they resolve. Effects that switch the creature's power and toughness are always applied after any other power or toughness changing effects, including these two, regardless of the order in which they are created.