Text (AL+errata): 1/5. ; {B}: ~this~ gets +1/-1 until end of turn. ; {1}{U}: Switch ~this~'s power and toughness until end of turn. [Oracle 2005/08/01]
As per Rule 613, power and toughness switching is applied after any effects that modify power/toughness. This means that even if new spells and abilities are played to modify power/toughness, they are applied before the switch. [D'Angelo 2006/01/01]
Note - This card was of creature type "Phantasm" and not "Illusion" until Oracle 2006/07/14.