You can easily move the maildirs by renaming them, but you want to do this before you start receiving emails on the new domain name so as not to overwrite new emails. This assumes that you have a working knowledge of linux and are able to use and understand the
mv command. This however only migrates your emails. As you have created new email user accounts, you will have to have each user redo his personal settings.
If you already have received/sent emails from the new domain (and cannot or do not want to overwrite them) then you will need to use one of several manual methods of copying the emails over. You can use the email migration tool imapsync, or set up both users in a desktop email client and drag/drop emails from the old account to the new one.
A possibly simpler option would have been to create a domain alias which would have sent all emails addressed to example2.com to the mailboxes on example1.com. This may have been less desired in the long run though as users would still login to their email accounts with the username of email@example.com.