I got an item last session from my GM and lost my char sheet and am now trying to find the weapon in the compendium.
What magic weapon has the ability to change into any other weapon and to be changed back once per encounter?
You're looking for a Dynamic Weapon
Power (Polymorph) Encounter (Minor Action) Change the weapon into a different weapon from any melee category (simple, military, or superior). This effect lasts until the end of the encounter, or until you end it as a minor action.