Pre-migration

I see a few sets of migration instructions, but nothing on what to do outside of the ZeXtras-specific migration of the settings and data.

Going from multi-store7.2.x CentOS to multi-store multi-master 8.6 Ubuntu LTS.

Guess:
1) Install the server infrastructure as you would like the final setup to be including the number of MTAs, proxies, mailstores, LDAPs, etc.
2) This would be on new IPs and rDNS.
3) Shorten public DNS TTLs.
4) Proceed with appropriate Zextras migration, including moving public DNS records at an appropriate time for your migration method.
5) Decommission old servers.
6) Add old IPs to new servers if desired.
7) Change DNS to old IPs if desired.
Retire new IPs, if desired.