Windows 8.1 is a free upgrade to Windows 8. The edition does not matter so much, just the version (i.e. - PRO doesn't mean much, Windows 8 or WIndows 8.1 does) in your situation.
You had Windows 8 PRO. You get Windows 8.1 PRO for free. Depending on how Windows 8 was originally put on the machine (legitimately, of course) would determine whether you have a product key you can copy on paper and put in a safe place or if it is embedded in the system BIOS.
So, if you had Windows 8 PRO and it was a legitimately licensed copy - your Windows 8.1 PRO upgrade was (always will be) free and should not have required any action on your part.