Some game features, particularly feats, give you the option of swapping one of your adventurer’s powers for a different power. This option provides a way to customize your adventurer and to experiment with different abilities.
You might be given the option of swapping a class power for a power that is not from your character’s class. You cannot make the swap if doing so would eliminate your character’s last class power of a particular type: utility power, at-will attack power, encounter attack power, or daily attack power. For instance, if your wizard has only one wizard utility power, you cannot swap that power for a nonwizard power. Unless instructed otherwise, you cannot replace powers that your character gained from a paragon path or an epic destiny.
Published in Rules Compendium, page(s) 90.