CD keys are bound to the CD's themselves.
I think the upgrade version is only for upgrading a different version of Vista, for example if you had Vista Basic and wanted to upgrade to Home Premium then you wouldn't need the CD Key.
I would go and get a replacement that isn't the upgraded version (it costs a little more but you get the CD key with it) otherwise you will have just wasted about £70?
If your worried about losing files, create a seperate partition to the one your going to delete and backup any files and folders you deem important and worth keeping, then go from there.
