Rule 601.3b

If an effect allows a player to cast a spell with certain qualities as though it had flash, that player may consider any choices to be made during that spell’s proposal that may cause that spell’s qualities to change. If any such choices could cause that effect to apply, that player may begin to cast that spell as though it had flash. Example: An effect says that you may cast Aura spells as though they had flash, and you have a creature card with bestow in your hand. Because choosing the bestow ability’s alternative cost causes that spell to become an Aura spell, you may legally begin to cast that spell as though it had flash.