The RC is not the finished product, is still buggy and has a ton of monitoring software taken off in the RTM. There is a blog post from a Windows 7 developer sticky'd on Technet Install forum begging users not to upgrade it. Some have, though.
That said you can still affordably achieve it with this fix: UNAWAVE - Downgrade Ultimate to Home Premium or Professional
Note the extra caveat at the bottom for using the RC.
Your best bet for Up/downgrading is the $150 family 3 pack of Premium upgrades, less than half price each which is the best deal offered yet. You can have it on 3 machines of your choice for life.