Your old Exchange Server should be decommissioned when you are upgrading or switching to another Exchange Server. A proper plan is required before going ahead. All the content of User Mailboxes and Public Folders have to be migrated to the Target Exchange Server. Keeping in view the different issues that may arise while decommissioning the Exchange Server, we have created three documents to illustrate the step-by-step process to decommission Exchange Server 2003, 2007, and 2010.
You can click the following links to access these documents.