I think majority of the people here are going to reinstall a clean copy of the RTM build once it is announce. Some people may already have the supposed RTM build, and will just keep on using it until their software ships/becomes available, then they will input their key and activate Windows 7.
You do make a valid point though, that you cannot downgrade a system from Ultimate. This is something people need to keep in mind too. If they are installing the supposed RTM build and selecting "Ultimate" as the version, when their copy arrives and it is a Home Edition, they will have to rebuild.
I think the majority of the people here are conformable rebuilding their machine and will happily do so when the final products ships/becomes available.
I see you mention upgrading an RC to an RTM, which is something I cannot recommend. I am sure someone has the process documented, but I would suggested starting out with a nice clean slate on a nice new operating system. When it is officially release, I suggest installing it from scratch.
Hope this answers some of your questions.