ok I did it!! I had to follow a guide to uninstall both licenses using the command prompt so the licenses can be used later. Then used add features of win 8.1 to upgrade my main pc to win 8.1 pro using the pro key. The htpc downgrade was more problematic at first but then found a little program called updown8 that changes some registry values which allows the use of a windows 8 key from any version. Here I had to use an installation media (I used usb) to "reinstall" windows 8.1. This time the installer accepted my key since it was not being used anymore.