Nice post from Mohamed Marzouk (MSFT) where he explains a high level overview of a transition from GroupWise to Exchange Server 2010.
The following steps summarize the migration process from GroupWise to Exchange 2010:
1. Design Exchange 2010.
2. Implement and configure Exchange 2010.
3. Select the migration software (3rd party).
4. Setup Co-Existence between GroupWise and Exchange.
5. Migrate the users’ mailboxes.