We probably should, somewhere. I don't know whether it would be best in the 'how do I switch from Plus' FAQ, though, or in one of the Comms faqs, or both.
So for location, I leave it open for suggestions. Text could be something like:
"To switch your community to or from the Plus level, go to the [Community Management] page while logged in as the maintainer. Click the Account link for the desired community and use the appropriate button."
Or whatever works better. But I do think it should go somewhere.