Ahn-Crop Champion

All attackers are chosen at once. You can’t attack with Ahn-Crop Champion, untap a tapped creature, and then attack with that creature.

Wotc — April 18, 2017

Untapping an attacking creature doesn’t remove it from combat.

Wotc — April 18, 2017

If you attack with two Ahn-Crop Champions and exert both, each will untap the other.

Wotc — April 18, 2017

You can’t exert a creature unless an effect allows you to do so. Similar effects that “tap and freeze” a creature (such as that of Decision Paralysis) don’t exert that creature.

Wotc — April 18, 2017

If an exerted creature is already untapped during your next untap step (most likely because it had vigilance or an effect untapped it), exert’s effect preventing it from untapping expires without having done anything.

Wotc — April 18, 2017

If you gain control of another player’s creature until end of turn and exert it, it will untap during that player’s untap step.

Wotc — April 18, 2017

All cards in the Amonkhet set that let you exert a creature let you do so as you declare it as an attacking creature, as do some of the cards in the Hour of Devastation set. You can’t do so later in combat, and creatures put onto the battlefield attacking can’t be exerted. Any abilities that trigger on exerting an attacking creature will resolve before blockers are declared.

Wotc — April 18, 2017