Yes, I accidentally kicked off the Digital River student discounted Upgrade version on my Win 7 RC desktop in the wee hours of this morning while testing to see if the bootable disc I created worked. Since I was planning on doing this soon anyhow I didn't worry about it kicking off other than I lost an hour more of sleep.
In my case I was moving from x86 Win7 RC to x64 Win7 Pro. The installer recognized an upgrade-in-place couldn't be performed (which I believe was discussed here since the RC version is technically a beta version) so the installer backed up my Program Files, User, and Windows folders to the "Windows.old" folder. I do have to reinstall all my programs but any program settings in the USER folder are retrievable.
Oh, and it activated smoothly.
edit: I imaged the drive after activating and updating Windows so I can restore from the complete image rather than have to reinstall, validate using a prior Windows version (if need be), and activate again.