Downgrading Windows 8 from Enterprise to Pro

Solution 1:

Yes it is; it just takes a little time (it is worth it)

1. Open registry editor (run regedit.exe) and navigate to HKEY_LOCAL_MACHINE→SOFTWARE→Microsoft→Windows NT→CurrentVersion
2. Double click on ProductName and change to "Windows 8 Professional".
3. Double click EditionID and change to "Professional":
4. Now navigate to HKEY_LOCAL_MACHINE→SOFTWARE→WOW6432Node→Microsoft→ Windows NT→ CurrentVersion
5. Do the same as you did in steps 2 and 3.
6. Close regedit (You don't have to restart your computer).
7. Close and insert Windows 8 Pro CD and install.

Editor's note: Somebody who didn't have commenting privileges tried to edit this answer with the addendum "You do need the install files and a product key for Windows 8.1 Pro."

Solution 2:

No, you can not downgrade from enterprise to pro without re-installing windows. (also it would be a upgrade, as enterprise and pro are identical, except pro also has media center)

If you are trying to go from Enterprise Eval to something else here is some more information about it.