Rule 508.2 · Declare Attackers Step

Abilities that trigger on a creature attacking trigger only at the point the creature is declared as an attacker. They will not trigger if a creature attacks and then that creature’s characteristics change to match the ability’s trigger condition.

Example: A permanent has the ability “Whenever a green creature attacks, destroy that creature at end of combat.” If a blue creature attacks and is later turned green, the ability will not trigger.

Any abilities that triggered on attackers being declared or that triggered during the process described in rules 508.1 are put onto the stack before the active player gets priority; the order in which they triggered doesn’t matter. (See rule 603, “Handling Triggered Abilities.”)