What happens when I reprioritize a message in Campaigns if it is already in the process of sending?

Reprioritizing a Campaign that is already sending will only affect those users who have not yet received that specific message. Like all messages, users who have already been sent a message will not be sent it again, even if it is re-prioritized. The next time your users are evaluated to identify profiles that qualify for your Campaigns, they will be sent a message based on the new prioritization.