How to plan Active Directory migrations

Active Directory migration from an older version to a new version is a common challenge for identity infrastructures. In some documents, this is called Active Directory upgrades, but I prefer to call it Active Directory migration. In this process, a new AD DS version will install in a new server or an existing server. Then the FSMO roles will migrate to the new domain controllers. Once it's completed, the older version of AD DS will be decommissioned. Afterwards, the domain and forest function level will be raised to match the new AD DS version. Even though each AD DS version has core functions that are the same, newer versions always have new features and enhancements which apply at domain or forest level. Therefore, it's more of a migration than an upgrade. The migration term is also used for migrating Active Directory objects...