Changing their form without resealing their powers in a sword is the same as burning off an arm, and if they were to discard part of their released form, while in it, they can never return to normal again. The only time they can return to their humanoid form is when they reseal their powers in sword form. When Resurrección is completed, the Arrancar's prior injuries are healed. The Zanpakutō doesn't need to be fully drawn out from the sheath to activate Resurrección. When they release their weapons' seals, they unleash their true power and their true form. Arrancar usually seal the nuclei of their abilities within the form of a sword, which is entirely different from what the Shinigami use.