have dealt with M$ products for many years and I suspect the key for the 32 bit OS will not match the algorithm key designed for 64 bit. It is a hunch however I say this because during the days of WinXP 64 bit a 32 bit key would by no means register.
To be sure I would contact Microsoft however your 64 bit version of the O/S will come with a new key which you will need for the install.
Of course this is speculative however a call MS support most probably (India) will set you down the right path and clear up any confusion.
BTW hope you haven't registered the 32 bit version yet as you would want to know what happens to that key after the move to 64 bit. You don't want that key bound to you 64 bit rig if you are looking to upgrade/reinstall the 64 bit OS.
i just bought vista home premium. i thought it came with 64 bit os but it doesnt. so im about to order it in. now if i install the 32 bit os can i clean install with my same product key the 64 bit once it comes in ? i didnt install anything yet and yes my system is 64 bit.