Ordered wrong version (want 64-bit)

I just ordered an upgrade version of Win7 Professional 32-bit which is already in transit. But I'm having second thoughts about it and think I made a mistake here.

My understanding is the windows key will still work with either version and the only difference is the install disk. I have not yet received the product in the mail yet.

What's involved with changing this? All of my hardware is less then 2 years old so I fully expect to be able to use 64-bit without issue.

BTW I'll be upgrading from a 32-bit XP Pro.

EDIT: I found what I was looking for. Apparently you can download the 64-bit iso. Perfect!!

Make sure to run the windows 7 upgrade advisory if you haven't already. This will help you determine what hardware and software compatibility issues you might have.