I agree, Bounty managers has the right to change rules, unless stated on the bounty rules in fact when the team decided to change rules especially for KYC and all that stuff, it might be okay because it is one of the requirements nowadays in order to find those spam accounts, which is proliferating nowadays.
It's still better if the original rules will not be change after the token or coin sale.
The rules or terms of the campaign is what makes bounty hunters applied, so they find the terms attractive and changing it like reducing the reward might not make them feel good, I mean, they give their best in promoting the project because they believe when it will become successful they will get the reward stated and it's a win-win between them and the project.
Bounty hunters can promote the project but they can also campaign against the project if they feel they are scam with the change of terms.