Convert mailbox (of domain) to domain alias?

[strarsis]

A new domain should be used as the primary domain for a website, the domain previously used for the mailbox should become a domain alias instead, so emails to the old domain should still be received by the mailbox and send from. How can this be accomplished? Do I have to manually copy/move all the contents of the mailbox (mail items, address books, etc) from the mailbox of old domain to the mailbox for the new domain?

[stale]

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[strarsis]

So I used the docker-mailcow sync feature (IMAPsync) to sync the contents of the old mailbox to the new mailbox. This of course also works from the same mail server to itself. Afterward, I set up a mail redirection in Sogo as I already do for clients that insist on some external email redirection.
Now I can remove the whole old mailbox and create an alias domain. The short time gap where there is no mailbox to receive the email to the old mailbox should not be an issue as the other MTAs retry and delay.