Anyone ever tried a downgrade?

[Toni S <tstonisaim@googlemail.com>]

Hi folks,

I have lenny running on a headless virtual server being hosted by a
small hosting provider and now I'd like to upgrade to squeeze. The box
does not serve any commercial purpose, but it does handle a portion of
my private email traffic. So I'd like the system not being down for
more than 12 hours.

In case of any service failures after doing 'sudo aptitude safe-
upgrade' I'd like to know some methods to smoothly revert to my
current installation.
Best would probably be to ask my hosting provider to make a disk
snapshot / disk image, but I'd like to know if there are other
possibilities. What's your experience? Did you try out something of
these:

- Using undo mechanism of aptitude (how does this work?)
- using 'dpkg-get-selections >currentstate.txt' and 'dpkg-set-
selections <currentstate.txt
- making a tar backup and restore it afterwards while the system is
running, then reboot?
- making small incremental upgrades of individual packages with
aptitude instead of upgrading all packages at once / test the new
service / revert to old version if necessary

(The latter is the way how i've done it with the previous 2 debian
releases)

What's your recommendation / experience for downgrade?

Thanks for any thoughts
T.