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?