User blog comment:Smarty Pants 264 Clash Royale/New Spell Idea 3/@comment-31334072-20170407140305/@comment-29396184-20170408123855

You will know because the transform spell will transforms troops or units into other troops or buildings that have the same cost. e.g You can transform a Giant into a Witch (because they both cost the same) but you cannot transform a Giant into a goblin (because they have a  different cost).