What should i say? Migration to squeeze worked like a charm. I changed my apt sources like this:
then did the magic trick:
aptitude clean; aptitude update; aptitude install linux-image-2.6-amd64; aptitude install udev
installing newest kernel and udev versions is suggested in squeeze release notes. After that i did a reboot.
installed all updated packages. I did another reboot after this.
installed grub2 into the MBR and i was done. A last reboot was necessary to check, if grub2 boots well and then i opened the bottle of beer i already prepared :-)
Just one thing is left and this is the migration from Courier to Dovecot :-(