How to easily move to a new mail server?

Question

postfix

The task was to move from the old mail server with postfix + dovecot + fail2ban + phpmyadmin + clam to a new one with iRedMail (everything is also only new) with a different white ip. I installed iRedMail on new hardware with a test domain. Checked the letters go and come everything is fine. Now you need to transfer all the mail and accounts of the old server to this server, and turn off the old one. The domain has about 400 accounts and a bunch of mailing lists. It is clear that it is necessary that in the morning users turn on their computers and do not understand anything, but everything was already ready.
My ideas about this:
1 Option.
1. You need to copy the certificates from the old server to the new one
2. Transfer the mail database with users and aliases through phpMyAdmin
3. Copy all mail from the old server to the new one at night.
4. Change A record in DNS
5. Turn off the old server and be afraid.
Option 2. Since there are two servers and I have an IP for each, as I understand it, you can turn on both of them and let two work for a while. Only the letters should still be on a new one. What in this case with certificates?
Thanks