Win10 does not alter your bios in any way. If you needed to revert, the key will still be in your bios to re-activate your old OS.
Then don't upgrade, it's optional. Better yet grab one of the ISOS here and install it in a VM and play around with it. If you don't like it it's one click to delete it and if you do you still have a year to upgrade for free.
has anyone had success with upgrading through winupdate this morning,i woke up and had win 10 failed in winupdate history,i didn't know it would try and install by itself,i thought we would have at lest a button to push,like "win 10 is ready to be installed",i have gone ahead and am trying to upgrade with a disc.