[gloria_mundi]

I did exactly that about a year ago (except it was testing instead of unstable). You need to pass some extra flag to APT because Ubuntu and Debian use different version schemes, so you wouldn't get updates anymore -- might have been '--reinstall', but don't quote me on that.

It worked quite well for me, although there were a few issues (for instance, I didn't get kernel updates anymore, and when I fixed that, the Wifi driver was missing). There are probably still a few issues that went undetected and might cause problems later.

It's certainly possible, but I wouldn't recommend it unless you're willing to spend quite some time on it. And don't count on it working at all. (And backup your data. I didn't, and got lucky, but in retrospect, that was irresponsible.)