Well, That Was Easy (Bookworm to Trixie)
Server upgrade from Debian 12 (Bookworm) to Debian 13 (Trixie) took 40 minutes.
The upgrade had been postponed for long. Three years ago, moving from Debian 11 to Debian 12 I spent a few stressful days to reconstruct my mail¹
Things went very different this time: the entire upgrade took just 40 minutes. After the update, and following the reboot, all it took was one extra line in the Exim conf, and one new systemd service.
Debian: thanks!
- One of several bugs that had tripped me up at the time, now has this super easy fix:
mailbox_legacy_dirs: yes