Is there an easy way to get back from unstable to testing?

[Stephen Rueger <stephen.rueger@hadiko.de>]

Hello!

I made a silly mistake and did an apt-get upgrade and forgot that my
sources.list was pointing to unstable instead of testing. Then I made
a even more silly mistake and didn't looked at what apt-get was
doing. So now I have an testing/unstable-installation that is really
unstable (e.g. fetchmail stops because of a segmentation fault).

I tried to find a way to downgrade back to only testing, but the only
way I found would be to remove all packages not needed to install
other packages and then doing a nearly complete reinstall. 
apt-get --reinstall doesn't work in this case because I have to
replace packages with older versions of it.

Can someone give me a hint how I can do this with less work of my own?

Thanks!

Stephen Rueger