When designing an Active Directory infrastructure from the ground up, it would be ideal if management can confirm what it will become in 20 years' time. Then, you know what to do. But this only happens in a perfect world. When organizational changes happen, it may affect the infrastructure too. It can be a change, such as introducing a new department, hiring more people, introducing a new business acquisition, or doing business mergers. Some of these changes may be easy to implement, and some may require larger, organization-wide changes. If we think about it from an identity infrastructure prospect, the most challenging change will be to extend the security boundaries. Your Active Directory forest is your security boundary for the identity infrastructure. If your requirement is beyond that, it always involves a design change.
For example, if Rebeladmin Corp. merges with My-Learning Inc., the management of Rebeladmin Corp. needs a centralized IT administration and sharing of resources between the two organizations. Both organizations maintain their own Active Directory forests. The My-Learning Inc. forest is beyond the Rebeladmin Corp.'s identity infrastructure security boundary. In order to merge them, we need to create a forest trust or domain trust relationship between the two infrastructures. With these changes, the Rebeladmin Corp. identity infrastructure will have new security boundaries. Their operations and security will also change accordingly.