With 8.1 Microsoft did a way with the upgrade versions of it and now only offer full, but you should be able to either
1) search for the Windows 8.1 Pro pack which will make it to pro (pack $139 VS $199 for full)
2) Inside Windows 8, go to Add features and it may list the 8.1 pro pack as well
I cant test this as my 2 machines are currently 8.1 pro