As far as I know the Upgrade option is only supported from XPSP2 to Vista RC1 or from Beta2 ( after applying all available Windows-Updates ) to RC1.
So Pre-RC1 to RC1 could be a no-go.You can try, but don't blame anyone if it's not working.
"how do I know which OS it is upgrading ?" -
it will update the Windows Version you are in the moment using, when you start Vista's setup-process.
Example : you boot XP, insert VistaDVD and choose update, than it will update XP to Vista.
If you choose another partition to install Vista on, it will not update, but REPLACE the possible residing Windows on that partition. For updating you MUST boot the previous Version and do it from within a working Windows.
SBJ
