It sort of depends where the something goes wrong. if the error occurs in the installation before you choose the drive to install win 10 on, then yes, you could just put the old motherboard back in.
But if the problem happens after you have formatted the drive that has windows on it, you won't have windows to go back to as formatting drive wipes windows.
99% of time it will install fine, the only problems that sometimes occur is if Windows doesn't have the drivers it needs for the USB itself (only fix is use another USB drive) or there is another hardware problem.
boot from installer
follow this guide:
http://www.tenforums.com/tutorials/1950-windows-10-clean-install.html
when you reach the screen asking for licence, click "I don't have a key" and win 10 will continue to install and then once it is completed, follow the link I showed above to reactivate licence.