That is a good idea . Just leave it as is and list the new drive as the boot priority drive in UEFI so the regular boot is to the newer drive . The mainboard likely has a boot menu available which allows the selection of the boot drive without going into UEFI. Sure, as noted, it will need to d/l updates and will not have recent apps etc.after a while. But if you plan this as a back up system, then regularly keep it up to date against the day FUBAR visits. When that happens, you just boot the back up and away you go. I manually update my back up systems once a month which is very feasible for a home personal computer..
As well, an advantage is that you have a disk available to test out new apps before installing them on the current drive. You will quite enjoy the security of a full back up at hand that you manually update from time to time. You can also use the extra space for storage.