How can I reduce extended time?

Question: I've given away some "supporter" services for some users to help launch my community.

I did it by extending the blog

Now some of those people want to start paying. Is there a way for me to roll-back the time that a blog is extended for?

And, if I temporarily "remove supporter status", will that deactivate all supporter themes and plugins active on their site? If not, then I could remove supporter status just before they are about to pay...but I don't want to destroy their site in the meantime.