Can I use a vista upgrade disk when updating from the windows 7 RC to Retail?

I have heard conflicting stories about what to do when buying the retail while you have the RC installed.

When I bought my computer it had XP installed and a Vista business upgrade disk with it. When I installed windows 7 I did a clean install over XP. I have heard that when changing from the RC to the retail if you buy the upgrade disk you will have to insert your vista disk to confirm that you actually apply for upgrade pricing. Can I use the vista upgrade disk for that while I have the windows 7 RC installed? Also, I don't know the product key for this disk and my computer has only one disk drive (so to put in this disk I would have to take out the windows 7 disk). Of course I have heard from some people that I can use the workaround here ( to go from the RC to the retail without using a vista disk.

So can I get the upgrade pricing or would I have to get the full pricing? I don't want to spend $200 bucks on vista home premium when I can spend only $50.

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