What is "downgrading a system"?
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 upgrade, so you can restore your backup to "downgrade" it !
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)