I don't know the answer to your question, but in case you don't get a suitable answer from someone better qualified than me, you might try having dovadm pull the data my making an imap connection to the older server like:
doveadm -o imapc_features=fetch-headers -o mail_prefetch_count=20 -o imapc_port=993 -o imapc_ssl=imaps -o imapc_ssl_verify=no -o mail_fsync=never -o imapc_host=mail.example.com -o imapc_user=mailu...@exmple.com -o imapc_password=123456 backup -R imapc:
