I now back up my windows 7 laptop onto a usb attached disk drive that has less than 50% free space.
(using Windows own system image & back up utility). What happens when I upgrade to windows 10 and
begin using its back up on the same disk drive?

Or do I need to get a new drive for Windows 10 back ups?
Of course, if Windows 10 can't read the Windows 7 back up, is there any need to keep it?
(Assuming a smooth upgrade and no need to revert to Win 7)