[Adrian Schneider]

I've always done this:

Create a Premium / Subscriber usergroup that has extra priviledges. I usually create some plugins to jack up their features and remove/lower some of the limitations (time between posts, max number of images, etc.)

On top of that, give them a shiny title to show off, and access to a private forum.

I'd avoid removing any permissions you have on the current registered group. Members generally don't like that, and see problems paying to get something they used to get for free. Instead, give them additional features, and treat it more like a recurring donation + recognition for being kind enough to donate.