[Dovecot] migrate 15K users to new domain name

I need to migrate 15K users to a new domain name, and plan to use "dsync mirror" in the transition phase. Could someone confirm that this should work:

Before giving users access to new-domain do a first sync to get all the stale data over:

for user in $old-domain; do
	dsync mirror $user@old-domain $user@new-domain
done

Configure sieve vacation filter to forward all messages from $user@old-domain to $user@new-domain, and notify sender of changed address.

Give users access to both new-domain and old-domain, and do a final sync.

for user in $old-domain; do
	dsync mirror $user@old-domain $user@new-domain
	dsync mirror $user@old-domain $user@new-domain  # twice in case the first was slow 
	drop all messages for $user@old-domain
	Leave notice message for $user@old-domain saying he should use new-domain
done

Wait a few weeks/months, and then drop all users@old-domain.

Does this look sensible?

-jf