dovecot - Sep 2020 - maildir -> sdbox migration command for _all_ accounts, with split maildir/CONTROL/INDEX locations?

in a recent post, sven made a clear/cogent arg for sdbox vs maildir 
>	While Maildir seems to be an obvious choice, it has some hidden
>	performance impacts, namely it relies heavily on information encoded in
>	the filename, which results in a big overhead in meta-operations,
>	because Dovecot needs to stat() every file in a directory to gather
>	those information.
i've initially avoided sdbox, assuming the index-file dependency to be an
additional failure pt,
but that^^ is somewhat compelling.

i'd like to migrate maildir -> sdbox.

the given migration example

	$ doveadm -D -v -o backup -u sven sdbox:~/sdbox

examples in the docs generally demonstrate a single user migration, from a
consolidated ~/Maildir tree, and use the 'dsync ...' command (full
alias?)

atm, my 'mail_location' is set with split CONTROL & INDEX locations,
in fs-layout, as

	mail_location =
maildir:/data/vmail/%d/%n/Maildir:CONTROL=/data/vmail/%d/%n/_control:INDEX=/var/vmail-index/%d/%n:LAYOUT=fs:UTF-8

and I've a significant number of domains & users.

I'd like to migrate the ENTIRE server -- all domain/user accounts.


For the moment, I'd like to ensure that I do not change/destroy the current
Maildir stores/indexes, and end up with a parallel store in

	mail_location =
sdbox:/data/vmail/%d/%n/Sdbox:CONTROL=/data/vmail/sdbox/%d/%n/_control:INDEX=/var/sdbox/vmail-index/%d/%n:LAYOUT=fs:UTF-8

which I can then double-check, and eventually switch to.



	What's the correct procedure/syntax for an all-accounts, in-place migration
command from maildir->sdbox?