702.102a Bestow represents two static abilities, one that functions while the card with bestow is on the stack and another that functions both while it’s on the stack and while it’s on the battlefield. “Bestow ” means “You may cast this card by paying rather than its mana cost.” and “If you chose to pay this spell’s bestow cost, it becomes an Aura enchantment and gains enchant creature. These effects last until one of two things happens: this spell has an illegal target as it resolves or the permanent this spell becomes, becomes unattached.” Paying a card’s bestow cost follows the rules for paying alternative costs in rules 601.2b and 601.2e–g.
610.3. Some one-shot effects cause an object to change zones “until” a specified event occurs. A second one-shot effect is created immediately after the specified event. This second one-shot effect returns the object to its previous zone.