Before you upgrade to Windows 8.1 you will need to return your Pro license as 8.1 will act as another install.

To prevent you from reaching the install cap, before you upgrade to Windows 8.1 you will need to;

Find your serial number (for later)
Got to "Manage License" in the Unity menu on Mac, in help menu on PC.
Click "Return License"
The editor will close and you will be able to input your serial number when you upgrade to 8.1.