Portal Mage

• WOTC Official Reminder Textofficial rules
  Flash — You may cast this spell any time you could cast an instant.

• WWizards of the Coast2017-08-25
  Official Ruling
  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.
• WWizards of the Coast2017-08-25
  Official Ruling
  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.
• WWizards of the Coast2017-08-25
  Official Ruling
  Reselecting which player or planeswalker a creature is attacking ignores all requirements, restrictions, and costs associated with attacking.
• WWizards of the Coast2017-08-25
  Official Ruling
  Reselecting which player or planeswalker a creature is attacking doesn’t cause “whenever this creature attacks” abilities to trigger. Notably, the Curses in this set (such as Curse of Vitality) won’t trigger while Portal Mage’s ability is resolving.
• WWizards of the Coast2017-08-25
  Official Ruling
  If you reselect which player or planeswalker an attacking creature is attacking, that creature is still considered to have attacked the player or planeswalker as declared, but it is now attacking the new player or planeswalker.

Rulings via Scryfall