Hello! I buy avast pro to my notebook, one year later my notebook died, so i put my avast on my desktop computer and works fine, but recently, I make the mistake to update to windows 10 but the drivers of my motherboard dont work so well on W10, I make a downgrade to windows 7 ultimate. And I afraid to install avast on windows 7 and lose my license, I still have 1 year.
Hi alan.rebello 
You can not lose your license, only it should ( or seemed ) not ( to ) be installed on more systems than you have paid for.
So I recommend you to remove the “old” Win 7 and Win 10 installations from your Avast Account :
Log in on https://my.avast.com and select ( any old ) Device/installation - Settings - Remove from Account.
Also you can re-download Avast! Pro and your license file when you need it to be installed again…
Greetz, Red.