Migrating using a single forest makes sense for many organizations, and a move to Office 365 can be an important goal. In mergers and acquisition scenarios — especially those where a company is consumed slowly over time and many large Active Directory forests are present — it’s often already part of an organization’s consolidation plans.
There are typically two approaches in a scenario where you migrate through a single forest, depending on the organization’s business situation. The first may be to keep the forest where the majority of accounts and complex integration are present and to migrate accounts into this forest. This works best when the forest is already on a recent Exchange version, such as Exchange 2010 or Exchange 2013, since the components in a hybrid Exchange deployment are already present.
The second approach is often used when there’s no good candidate to serve as an overall company Active Directory (AD). This could be due to a merger and a group has a new name, but neither forest is named accordingly. There are ways to solve that issue, but it’s common to have the combined IT departments pool their respective knowledge to start with a fresh AD infrastructure and move accounts.
The approaches are the same for a hybrid Exchange deployment, whether it’s a new forest or using the best candidate for the final resulting forest. Each account is migrated using a tool such as the Active Directory Migration Tool, and then it’s prepared for the cross-forest move using Prepare-MoveRequest.ps1 script in Exchange 2010 and higher. At this point, DirSync creates a corresponding mailbox in Office 365, and the mailbox can be migrated to Office 365 either by first moving the mailbox to the target forest or by creating a migration endpoint in the source forest.