Backup, as simple as that. When planning a replacement for a production server or changing the configuration on a system that provides essential services for many users, the first line in our procedure plan needs to guarantee up-to-date backup. This is not a rollback procedure because a rollback may have a restore procedure as a step, but normally we are referring to two different tasks. We have many scenarios where we can execute a rollback procedure that does not restore data from the backup.
Do not delete any data; the ability to quickly and consistently recover the system to the last operational state, where users are not prevented from using the services they need, is an established best practice that we need to pursue while planning and implementing the replacement of the AD DC or any other critical network service.