I found this from Paul Thurrott site.

Yes, there is. But you can’t do it electronically. Instead, you will need to purchase the retail Windows 8 Pro Upgrade media, which is currently $30 more expensive (or $69.99) than the electronic upgrade. This package provides both 32-bit and 64-bit media, so you can choose which to use. (In case this isn’t obvious, you can’t actually do an in-place upgrade from a 32-bit version of Windows to a 64-bit version of Windows 8. Only clean installs (“Custom” install type) are supported.