I hope I did this maillist reply correctly. I am doing it with webmail, not an email client.

> libpam-systemd depends systemd-sysv. libpam-systemd also depends
> systemd. Devuan's libpam-elogind provides libpam-systemd but conflicts with
> systemd which must be removed first.
>
> However, to get systemd uninstalled we first have to replace systemd-sysv with
> sysvinit-core (leaving systemd installed) and reboot. systemd and elogind
> conflict so we cannot use libpam-elogind to replace libpam-systemd *yet*.
>
> This is the point we need a transitional package to temporarily satisfy
> libpam-systemd's systemd-sysv dependency whilst the system is rebooted.
>
> After reboot, systemd is no longer PID1 so can be removed and we can then
> complete the transition replacing libpam-systemd with libpam-elogind, removing
> systemd and installing elogind.
>
> I hope that is clearer.
>

I can get the general grasp of this information as I think a similar shim was used in devuan when it was first forked.

What I found to get sysvinit-core installed was to
-boot into the debian install, and before changing the sources list to point to devuan,
-run "apt install sysvinit-core"

then, alter the sources.list to point to beowulf and,
"apt update"
"apt upgrade"
"reboot"

No errors were noted and upon a reboot, sysvinit is running as PID1 and the migration can continue in a manner similar to what it has been in the past.

Trying to install sysvinit-core after the sources.list pointed to beowulf caused many problems and I never could get it done that way.

I documented my test migration of a buster Xfce install at:
https://dev1galaxy.org/viewtopic.php?pid=17716#p1771<https://dev1galaxy.org/viewtopic.php?pid=17716#p17716>6<https://dev1galaxy.org/viewtopic.php?pid=17716#p17716>