Acolyte's Reward only checks your devotion to white as it resolves (not before, not after). So the amount of damage that will be prevented/dealt is locked in at that moment.
Magic Rules Tips
…and knowing is half the battlefield
…and knowing is half the battlefield
Acolyte's Reward only checks your devotion to white as it resolves (not before, not after). So the amount of damage that will be prevented/dealt is locked in at that moment.