You can't, to go from 32 to 64 bit you need to re-install Windows clean.
But either case, you can't run an upgrade directly from XP to Windows 7 anyway. Unless you have your own licence of XP to install on that new computer, you can't just move the XP setup from your old computer to a new one. If you are building your own new system just get the Windows 7 OEM license.
If you are asking if you can buy a Windows 7 64 bit Upgrade edition and legally do a the setup of it from I do not think you can do that. So if you want to do the Upgrade to 7 64 bit, you would have to at least have the XP 64 bit media.