Hi,
attached you will find a proof of concept script for
a buster to beowulf migration. It is tested only in a
standard install (plus gnome DE and ssh) buster Virtualbox 6.1 vm
fully updated to version 10.9.

USE AT YOUR OWN RISK!!!
DON'T USE IN PRODUCTION!!!

Bold testers, hints and improvements are welcome.
Some hacks are ugly (I know but didn't knew better ones).
To test:

copy migration.sh to the new system
chmod +x migration.sh
# as root
./migration.sh

I've tested it from within the DE but it
could be used from the console (I suppose).

It works in two stages with minimal interaction.
After reboot remove references to debian buster
from /etc/apt/sources.list
and run:
apt purge -y systemd
apt install --reinstall elogind
apt update
apt upgrade
apt dist-upgrade
apt autoremove
apt install slim (and select slim as display manager)
reboot

Enjoy.

Ciao,
Tito