Portal Mage

If an ability targets something controlled by the “defending player” of an attacking creature and the defending player for that creature changes before that ability resolves, the ability won’t resolve because its target has become illegal.

You may cast Portal Mage outside of a declare attackers step. If Portal Mage enters the battlefield outside of a declare attackers step, its ability simply doesn’t trigger.

Reselecting which player or planeswalker a creature is attacking ignores all requirements, restrictions, and costs associated with attacking.