There is a
modification you can make to the CD to allow this, however it is not recommended that you do so as it was not designed for this and you may end up with bits of the RC left around to screw you up in the future.
Personally I would see if you can obtain the final release copy that is floating around now (eg. if your work have a technet or MSDN subscription, you can download it directly from Microsoft). It's Win7 Ultimate, but follow the procedure I linked above, but instead of mucking about with cversion.ini simply delete \sources\ei.cfg from the CD and then when you install you will be prompted to select what version you want, so pick Home Premium or whichever version you have pre-ordered and do not enter a product key when prompted.
You only get 30 days to activate, but just before the 30 days run out, open command prompt and type "slmgr /rearm" and it will reset the counter back to 30 days. You can do this up to 3 times giving you a limit of 120 days, which means when you get your final copy in October you can just enter your product key and you're sorted.
Edit: I just realised you're planning to buy an Upgrade copy.
Under no circumstances can you do an in-place upgrade from Windows XP. The "upgrade" edition is simply a lower priced version that requires Windows XP (or Vista) to be installed on your hard drive and activated before it will accept your product key. The advice I posted above will probably not work if you buy an upgrade edition