I have installed Windows 7 RC on my pc that was running XP Pro 64. Will I have to reinstall XP in order to do the upgrade to Windows 7 Home Premium?

Yes....I believe so.

Farmer Jack,I've seen both answers on the forums so far. In older versions, you could do a full install from the upgrade disk and just put in the original disk when asked. And I've read that Windows Vista didn't allow this to happen, had to be an upgrade of a working OS.

Farmer Jack, All signs so far point to your having to reinstall Windows XP before you can upgrade to Windows 7, as was the case when upgrading to Windows Vista - see this excellent ArsTechnica article.-Alex