Bind. Over several upgrades I have always kept the old config files. It seems that some ancient deprecated options now throw an error not a warning. systemctl start followed by journalctl -xe details all the problems, even the line numbers in the files so it was a matter of minutes to fix

Wireguard still seems to need the unstable repository, so changing that back it all worked, the conf file was still there so OK, same could not be said of NFTables, had to reload conf file from back up.

Then the 3 bigger ones:-

Roundcube. During install it said it had to reconfigure the database. I will have to purge, drop that database and reinstall from scratch unfortunately
Dovecot. My initial setting up, a few years back took ages (I was learning as I went) so I said no to replacing conf files. Had same problem as with Bind, setting which before led to a warning, now stop it from starting. Same trick as before and only one setting to change. The error message even tells you what to change it to. I should have heeded the warnings before. But it does mean that although Dovecot is delivering the mail to the boxes, I am unable to log onto Postfix as it uses Dovecot to verify credentials. But then my webmail uses Roundcube so I can't get at that mail at the moment anyway
Webmin. I use this as a graphical interface when working with big databases or updating and cleaning up all my zonefiles. It's just easier. Handy for editing Apache virtual host files. I was able to install it and start it then the connection drops to the miniserv server. I think it is related to an upgrading of the perl libraries in the upgrade. Did not have the same with my home boxes a couple of weeks ago. That is non urgent though.
Hopefully some lessons learnt so mistakes won't be repeated during upgrade of my other VPS tomorrow. Most of these irritations would probably not have arisen if I had cleaned up the conf files beforehand.
Hope there is something useful there for anyone else upgrading

Keith