I have tried twice now to upgrade Squeeze in-place to Wheezy, using the howtoforge tutorial. It works fine on generic installs, but seems to break things on complex servers. My first try was on a Plesk server, and it got totally confused. Then I tried it on a Perfect Server Squeeze/ISPconfig instance, and it broke authentication in some mysterious way: I just had to roll it back after wrestling with it for four days. The Courier/saslauthdaemon stuff just stopped working. Very frustrating- particularly in light of Squeeze security issues. It would be great if someone could walk through making this work.