Q: What happens if my usergroup changes partway through the campaign?
A: You will be paid according to the usergroup you began the campaign with, not the usergroup you ended it with.
Each month the campaign resets when you re-register? If you change your forum member rank mid-campaign the payment won't be affected for just only that monthly period or for all the campaign?
I should reword that rule. Basically in short, you'll be paid for the member group you started your month as, not the group you end the month as. However, once you're paid, users can apply again for an open slot for their new member group.
So if you start your month as a Full Member and end a Senior Member, you'll be paid 0.07
BTC for 50 constructive posts, which is the Full Member rate. However, when your payment is sent out, you can definitely apply for a Senior slot if one is open. If not, I'm usually willing to sign users up for a lower paying slot, and move them to a higher paying one, if one opens up.