Microsoft neither recommends nor supports an
"upgrade" of a Beta or RC1 system. There are
usually too many changed files to accomplish a
successful "upgrade."
So far beta to beta upgrades have not been supported. Beta to RC1 may be
enabled but Microsoft probably has little interest in that type of upgrade
since that scenario will not occur once Vista is released to manufacturing.
Microsoft is much more interested in how updates from W2k SP4 and XP will go
and are requesting testers to try it. If you are planning on upgrading
right through to the released version, don't.
Your Best Bet is to Re-Format and Reinstall Windows XP Service Pack 2 and
then Upgrade from Windows XP Service Pack 2, However my Personal Preference
is to Always Clean Install (especially if you plan on submitting Bug
Report's to Microsoft using the Microsoft Beta Client, to which the download
link from Microsoft can be found on Zack's Vista Base Website), just FYI.