I have spent the better part of 2 days trying to get Windows 7 Enterprise N to upgrade to Windows 8 Enterprise N with no results. The error message I receive after selecting the upgrade option and the subsequent compatibility check is:
Windows 7 Enterprise N cannot be upgraded to Windows 8 Enterprise N. You can choose to install a new copy of Windows 8 Enterprise N instead, but this is different from an upgrade, and does not keep your files, settings, and programs. You'll need to reinstall any programs using the original installation discs or files. To save your files before installing Windows, back them up to an external location such as a CD, DVD, or external hard drive. To install a new copy of Windows 8 Enterprise N, click the Back button in the upper left-hand corner, and select Custom (advanced).
I have tested both production and MSDN copies of Windows 7 Ent N > Windows 8 Ent N and neither will upgrade. Per MS documentation, I SHOULD be able to upgrade. I've found nothing regarding this via search engine except one person having the same issue when Windows 8 was still RTM. However, no one replied back to him.