Problems were caused by combination of old conf file and the new conf.d files which were not removed on downgrade.
I re-upgraded to the Sid packages, taking the new maintainer's conf files when asked. Then had to configure from scratch. Finally have it working.
1. New and old config files cannot coexist in many cases and the choice for keeping old files, modified or not, is not legitimate. Always upgrade.
2. A conversion script or a dpkg-reconfigure dialog should be offered.
3. Those 993, 995 port addresses are different than the prevous defaults. These new addresses had conflicts, the old ones did not. Script should probably check this and the new config might be better with the old ports of one specified.
4. Documentation is now out of date. New way to limit protocols to subset, i.e. just imap and imaps, is not suggested in any of the conf.d files, I assume should be in the master or mail conf files. Not the end of the world.