I have to change main board in my desktop. - it means that the hard disk with CreO ( with existing student licence ) will cooperate with the new machine.
What have I do to keep my license after installing the hard disc?
if you are changing motherboard the you are changing network card, too (because it is integrated on motherboard). If this is the case then you have to request new license file at PTC. See:
FYI: License file is located in C:\ProgramData\PTC\Licensing directory. It is related to MAC address of your network card.