You don't need to setup replication for that. See https://wiki2.dovecot.org/Migration/MailFormat

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Aki Tuomi
Dovecot oy

-------- Original message --------
From: Daniel Miller <dmiller@amfes.com>
Date: 02/09/2018 04:14 (GMT+02:00)
To: Dovecot Mailing List <dovecot@dovecot.org>
Subject: online conversion using replication?

With a single server - and no intent to have a second server online at
this time - is it possible to use the replication service to keep a
"live" backup? Or otherwise perform a storage format conversion?

I'm presently using sdbox - and considering going back to mdbox though
without SIS.  My intent now is ONLY a 1-way backup, to be kept current,
and no clients will utilize the converted storage (unless/until I change).

I've successfully executed:

doveadm backup -u <username> -n inbox mdbox:/var/mail/<domain>/<user>/mdbox

For a few users - but reading the docs leads me to believe I can
automate this.  But there's no explicit example for this - so I'm not
sure what to set.

Initially I'm thinking of:

dsync_remote_cmd = doveadm backup -u %u -n inbox mdbox:/var/mail/%d/%n/mdbox

and if that's right - which other services/listeners do I need to setup?

--
Daniel