From RC-2 to Business Upgrade (MAPS)

Dear Group,
Sorry if this has already been asked but I couldnt find it in the search of
the last 1000. I am a MAPS (Microsoft Action Pack Subscriber) and had been
sent the RC-1 of Vista Ultimate and I installed it on a production machine
to start testing with.

I was sent out Vista Business Upgrade in the quarterly update that came out
yesterday. Because the RC-2 is Ultimate and the final version is Business
and Upgrade at that, can I install Business from within Ultimate? Or, do I
need to reinstall XP, then install Vista from within XP? If it is the
latter, shame on Microsoft for sending out the Business Upgrade versus
Business full edition..

Based on 1) not being able to install the upgrade outside of Windows and 2)
not being able to downgrade to Business from Ultimate, I am sure that I will
have to reinstall XP then upgrade to Vista, but I wanted to be sure. Thanks
for any help.

Andrew