Ok, the system upgrade is basically done. The only losses so far are the mailing list system - although that's mainly because I simply have no interest in running it anymore. In principle, it was completely updated, I just threw it out because I don't want to do anything else with it - there was only one list in it. And otherwise, mainly old junk has been thrown out.
However, after two system upgrades, I have to say that I'm not really enthusiastic about this upgrade - it already shows the problem of the extremely long release cycle. The first upgrade went through quite smoothly - the machine in question was one that already ran Sarge, just an old version from Testing and not the current Stable. The upgrade caused no problems.
The second upgrade, however, was simon.bofh.ms - a machine that was still largely on Stable, with a whole range of backports (self-made and from the net). The latter is of course the real problem - because the release cycles are very long, it is often necessary to install packages yourself. The Debian upgrade mechanism should still handle this. But reality shows that packages from backports often refer to intermediate states in which bugs in testing packages are present or simply special features that were not taken into account. As a result, a whole range of package upgrades were very tricky and I would not want to subject any normal user to going through that.
The highlight of all the problems was the PostgreSQL upgrade, which went through cleanly but then did not start due to an outdated option in the config. The messages were so cryptic that even I could not immediately see what it was - only digging in the logs and looking in the scripts confirmed to me that the upgrade was clean and really only the start had jammed.
However, I still have to say that the upgrade of a machine with partly up to 3 years old program versions went surprisingly well and 99% of the packages were updated completely problem-free - even things like my rather exotic Exim4 installation (a self-made backport with special features) went through quite smoothly - manual fixes were necessary, but I had caused them myself. The Apache and the whole PHP mess ran completely problem-free, the MySQL database also ran immediately. And one should also note that the whole upgrade - although described by me as suboptimal - only took 1:45 hours. And most of that was waiting for the packages to unpack ...
Well, in the next few days it will show what else has broken and which of the scripts no longer run that I have overlooked so far.