I only did this 3month ago , When i first upgraded my motherboard all was good and win 10 was activated np ,but then 2 days later my M/B died and i had it replaced with a new one about 5 days later but this time when i installed win 10 it wouldn't active with the key i was using which was my oem key of 8.1 ,so i go on to microsoft chat and after they tried at there end to active my copy they couldn't so they then took control of my PC from remote access and did a load of things but it still wouldn't active , the technician then went and got his team leader who after 10 minutes of trying to get it to active issued me with a brand new key and also told me that this new will work if i need to reinstall the o/s anytime in the future ...
I wasnt going to try the chat system at the time as i was going to just buy a new but i decided to give it a go and they where very helpful in the end and saved my a few quid to