I just tried upgrading 11→12 on a laptop of mine. The only "snag",
which was no surprise, was being left with a grub.cfg that only boots
the one, upgraded system. That was quickly rectified by grub-mkconfig
after uncommenting GRUB_DISABLE_OS_PROBER=false in /etc/default/grub.
(Thanks for putting that commented line into the file.)
Under bullseye, there were 2015 packages, including libreoffice and
texlive. I run fvwm (now called fvwm2, apparently) with no DE/DM.
It was "pure" Debian except that xtoolwait (from squeeze) was and
still is installed.
Running buster, I copied my bullseye root filesystem to a spare
partition, and adjusted the LABELs in the new copy's fstab.
I booted it up and edited the sources list to bookworm, including
adding the new non-free-firmware.
I ran apt-get update, apt-get upgrade, apt-get --purge autoremove,
apt-get dist-upgrade and apt-get --purge autoremove again, answering
no to keep my configuration files, yes to ignore any bug reports, and
yes to restarting services (including any that might have disconnected
a remote session). It was running on wifi, but I was at the console.
Statistics for the four steps: 627 upgraded and 972 not upgraded;
75 to remove and 968 not upgraded; 967 upgraded, 245 newly installed
and 13 to remove; and 79 to remove. Every thing completed smoothly,
with no dependency problems at all.
I haven't used it in anger as I don't want to disturb my dotfiles etc,
but I checked that sound played perfectly with timidity. Obviously
I've got a number of changes to read up on, with some new packages
to look over.
Cheers,
David.