If you don't have a backup of your old OS or the restore media, you won't go back with any ease.
If you didn't make backups or the restore media you might be able to order your laptop's restore media but a clean install of 10 has so far wiped out the factory and rollback to 8.
-> In parting I don't see any good reason to go back. Here I installed Classic Shell on Windows 10 and I'm sticking with that.
I had windows 8.1 and I did an upgrade to Windows 10 but know I want to do a clean install
Would I loose the chance of going back to windows 8.1 if do a clean install now?

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic