If you do the upgrade and want to fresh install, you'll have to do:
1. Go through with the upgrade so that it generates your hardware ID for activation (you don't use a key anymore for Win10 unless you buy a retail version of Win10).
2. After the upgrade completes, then you can either use the Media Creation Tool for WIn10 by MS or use the Reset PC feature under Settings -> Updates & Recovery -> Recovery
3. After choosing which you want to do, you can reinstall and not have to worry about activation since it's tied to your hardware ID and will auto-activate once connected to the internet. If you use the install media, you'll just have to skip the product key screen.