The method I did is as follows (in case it helps)
Upgraded from 8.1 - 10 using the official tool in the system tray
After upgraded I ensured it showed as activated
Clean installed by wiping my SSD and installing w10 off the media disk I created
It then activated automatically as soon as it connected to the internet as its tied to your motherboard.
As long as you upgrade and activate first before doing a fresh install you're fine. I also learnt the hard way as I reserved, made media disk, fresh installed before upgrade and it wouldn't activate so had to start at 8.1 and upgrade lol
In addition, Im changing my motherboard soon and will also be able to reactivate and clean install 10 straight away as I will give up my 8.1 retail key to msft and get a 10 one in exchange.