<snip>
>> If you want to keep sysvinit, here's the order of events:
>>
>> change sources from stretch to buster
>
> Fine.
>
>> apt update
>
> Splendid.
>
>> apt dist-upgrade
>
> Great.
>
>> apt install sysvinit-core
>
> What happens if this is not done?
Next time you reboot, you'll have systemd as init.
The Confusion behind this, is one reason why I am friendly to Devuan (
https://devuan.org/), while still running some systems as Debian (with SystemD).
To the Original Poster, I would only suggest to examine the Devuan Website, to see if it's an option. (Note, however, that they are, as far as I know, behind Debian by one Release, with their "Ascii" Release matching Debian Stretch).
The dist-upgrade will have resulted in installing the systemd-sysv
package, which (despite its name) has nothing to do with sysvinit; it is
the package which sets systemd as the primary / active / default init
system.
Installing sysvinit-core will uninstall that package.
Good luck!
Kenneth Parker