Dovecot index compatibility

[Frido Otten]

We're in the process of upgrading Dovecot from version 2.1.17 to 2.2.27
(package Debian Stretch). We have a shared NFS storage in use by
multiple servers without Dovecot director. I've tested by connecting to
IMAP on the new server to my own mailbox. This isn't a problem. But when
the server with the Dovecot version 2.1.17 LDA drops a new message in my
mailbox it thinks the index file is corrupted, and the message bounces
to it's sender. So there must be a index compatibility issue between
both versions. This is still no problem if I upgrade the entire platform
in a maintenance window at once. But what I want is a rollback
possibility. To which minimal Dovecot version should I upgrade the old
platform to create this rollback possibility.

I was looking for a table of dovecot index compatibility between the
different versions (forward and backward), but couldn't find it. Is
there any? Please advice.

Best regards,
Frido