I had no problem to do testing->unstable, but it highly depends on the day you do it, for example today, python2.3 fails to install (it took me 5minutes to find a workaround by looking at bugs.debian.org).
I have not yet tried the opposite way, but this may help you:
http://linuxmafia.com/faq/Debian/downgrade.html
(i hope its not outdated..)