cr:1.8.10a

If an object has already been played, activated, or triggered, or otherwise has come under the player’s control, and then the properties of that object change to become an attack as specified by the effect, the effect does not apply.

Example: Take Up the Mantle has the text “. . . you may banish an attack action card with stealth from your graveyard. If you do, the target becomes a copy of the banished card.” If a player plays Prismatic Leyline, then a red attack with stealth, and finally Take Up the Mantle banishing a blue card, the attack will not get +3{p} from Prismatic Leyline as it was already under the control of the player and is not considered their next blue attack.