Yes, you can roll back to 8.1 and reinstall 10. I did it with Windows 7. As long as you still have your Windows Old file from the upgrade you can roll back within one month of installing 10.
Rolling back is very quick (took 10 minutes for me). Be aware that 10 may leave a couple of things a little mucked up (it cause problems with Internet Explorer for me, which is why I ended up reinstalling 10), and they do warn that you may need to reinstall some programmes after the roll back. I didn't, but apparently it happens. You'll still have a year from when you first got 10 to reinstall it, and you'll need to reserve again.
After you've rolled back, you can cancel your upgrade, and hide the Windows 10 update in the meantime, to stop it nagging you. You'll have a few updates to install afterwards, and when you choose to reinstall , there will be more to download and install prior to upgrading again.