The man doveadm-sync
pages are cryptic and not very well explained, as well they are missing quality real-world examples.
This gist aims to give some clarity and explanation.
Here is the command I got to successfully transfer (and sync backwards too) an email account from an old Dovecot email server to a new Dovecot email server:
To my knowledge, both servers must have a matching account already setup for this to work: