I guess you could boot into a live installer ISO and get that to install over the root disk, though I don't know what problems you'd face with trying to unmount

I guess you could boot into a live installer ISO and get that to install over the root disk, though I don't know what problems you'd face with trying to unmount the original root partition. If you're wanting to do something like that fairly regularly, then it'd be easier to just run virtual machines (e.g. QEMU/KVM) to do that kind of thing.