If you can take a disk image of your drive as it is now you can try it out.
This gives you the ability to at least restore back to the win10 you have now.
MS gives you 30 days to try win10, during this time you can roll-back to Win8.1 (your original OS) from within win10. (I would still image 1st for protection)
The theory is that once you have activated win10 the key is associated with your machine via a calculated unique id.
You can do a clean install of win10 over your current build and the activation is still there. [Done this and it works]
If you are within the 12 month 'Free Offer' this should work. (Remember win10 Upgrade only free during 12 month period from 29th July, after it is a chargeable item)
I have not reverted to win7 and back to win10 so can only tell you what is supposed to work.
Cover your back by making a full image backup.
If you restore this at a point when Win10 is 'Better' it will automatically update to the latest version once it is running and on-line.