So we ran into an interesting scenario. We are playing as Armored Onslaught class cards from Return to Hoth expansion. It states "exhaust this card while the imperIal figure is defending and choose 1 of the following keywords: pierce blast, cleave, or any condition keyword. During this attack, the chosen keyword has no effect. Then, you may pay 1 threat point to ready this card." My question is after the Rebels choose their surge ability to use (example cleave), then ImperIal player chooses to counter with shields picking keyword "cleave" to have no effect, can the rebels change their choice and pick a different surge ability? It seems this can end in a never ending cycle because then the Imperial would say "never mind - I'll use a different keyword to choose". Can you change what you spend the surge after it's been cancelled?

Power to Shields is while defending, so it can be used just before/during step 7 of the attack. The attacker cannot rewind the attack steps to change how to spend surges.

Also note that Pierce is taken into account during step 7 of the attack, it doesn't actually cancel any blocks (removing symbols would otherwise happen immediately when the ability is triggered).

Ability resolution order during attacks:

https://www.boardgamegeek.com/thread/1631857/ability-resolution-order-during-attacks

Thanks for clarifying - that helps!

I don't know any game rules that allow you to change a choice after the opponent responds to that choice. "Oh you moved your pawn there? I take back my last move then".