What is "downgrading a system"?
In Debian world, it means to move to an older distribution (typically, to move from [:DebianUnstable:unstable] to [:DebianTesting:testing]). Do not expect to get anything stable by downgrading!
Is it supported?
short answer: No, it isn't supported.
long answer: It isn't supported because 1. The packages' installation scripts (postinst...) are designed to handled upgrade only. 2. The installation tools are designed to replace older versions of packages by newer versions.
So, what's the supported way?
- Make a backup of your system before you downgrade. If anything is wrong, restore your backup!
How to downgrade a system?
Be warned: you will be alone! However, Auke Jilderda wrote about the experience back in 2002: [http://people.debian.org/~osamu/pub/downgrade.html] (plus some comments from Osamu Aoki)