Upgrading Dovecot
The NEWS file, contained in the top-level of the source, contains all the important changes between releases. These changes are marked with * character.
Read them to see if there is anything that concerns you.
In general all the changes try to preserve backwards compatibility, but some changes which are meant to improve the stability and correctness of the configuration could mean breaking some existing installations. That may be a good thing, since it can expose problems which could otherwise show up as random errors.
Dovecot CE 2.3 and later
See the "Upgrading Docs" directory.