alfisya It is not a one way street, but it does have risk of incompatibel configuration leftover that can cause problem.
Thanks for your reply - I really do appreciate it. I guess I should have added "for all practical purposes," when describing the change to unstable as a one-way process.
I don't doubt that it's technically possible to return a computer to Shannon, given enough time, knowledge, and potentially debugging and repairing, as needed. But as I said in my message, creating the VM clone that I used required about 60 seconds of so, plus a little bit of configuration. And my laptop remains unaffected. Maybe I'm just lazy.