• Does the OS or Zimbra or Zextras need to be the same version between source and target?

Same OS on both servers is not necessary,  Zextras suite 2.6.3 is a dated version and out of support, in order to migrate to the latest version, I suggest you to upgrade to the latest Zextras or at least lagacy version which is 2.14.1 which is more compatible with the latest version, but no guarantee on migration.

• I can not (quickly) control DNS changes but I can afford some scheduled downtime. Instead of having both source and target "live" at the same time, can I make the backup to external storage (or NFS mounted drive), then shut down the source and do a restore in the target?

You may think about local IP address swapping, in case you do not have DNS access, NFS is not at all supported for default backup, NFS is supported only with backup on third-party features, so if you wants to use NFS you must migrate the backup to an NFS store by the `zxsuite backup migrateBackupVolume` command.

An NFS can be used to store data but network issues may interfere with RealTimeScanner and Zimbra/Zextras processes. Although Backup on a Third Party Store  may be supported, we usually do not it recommend for production environments to have NFS as the primary and only storage as having it temporarily unavailable would cause direct impact to users.

• Looking at How to install and update Let’s encrypt for Zimbra, the article assumes a fresh install but after my migration, will I also bring over the old certificates (with the now obsolete CA authority)? If yes, I can remove them from the system with Certbot but how do I remove them from Zimbra to install the latest ones as described on the article?

fresh install but after my migration is suggested. you can remove the SSL certificate by installing the self singed certificate.

• I have a Zextras license until April 2023, for the migration, I guess I can use the trial on the target. Is there a way to move my Zextras license to the target or do I need to purchase a new one?

I suggest you contact the sales team for the same.

1. Does the OS or Zimbra or Zextras need to be the same version between source and target?
2. I can not (quickly) control DNS changes but I can afford some scheduled downtime. Instead of having both source and target "live" at the same time, can I make the backup to external storage (or NFS mounted drive), then shut down the source and do a restore in the target?
3. Looking at How to install and update Let’s encrypt for Zimbra, the article assumes a fresh install but after my migration, will I also bring over the old certificates (with the now obsolete CA authority)? If yes, I can remove them from the system with Certbot but how do I remove them from Zimbra to install the latest ones as described on the article?
4. I have a Zextras license until April 2023, for the migration, I guess I can use the trial on the target. Is there a way to move my Zextras license to the target or do I need to purchase a new one?

Thank you for any help you can provide!

George