I wanted to replace Win7 RC with a full version, the RC was set to expire on 01 March 2010 and it had started to nag me to update.
Looking on the web it appeared it was possible to do an 'in place' upgrade (i.e. maintain all installed programs and files etc) with some tweaking of the settings, however this was also only possible if I installed Win7 Ultimate, which is kinda overkill for what I need.
In the end I went with Win7 Professional 'Upgrade Edition'. I assumed it would not let me upgrade from Win7 RC but I still had the original Visa Disks that came with the box when it was new, so planned to install that 1st and then upgrade to Win7.
As fate would have it no Vista install was required (thank god!). After booting to Win7 RC I put the new Win7 Professional Upgrade disk in and was able to do a custom install on the partition where Win7 RC was currently running from. The partition was not formatted, files and documents were moved to a backup folder (c:\windows.old). All that remained for me to do was to reinstall a few apps.
The whole process took less than 30 mins to get the new Win7 up and running.
When replacing Win7 RC the 'Upgrade' editions work just fine for a custom install.