There is a white paper available that describes how to change the service account safely with minimal interruption to Exchange Server services. To obtain this white paper, visit the following Microsoft Web site:
Important After much consideration, Microsoft does not support changing the Exchange Server 5.5 service account. If you experience a bug that is caused by performing the change, there will be no hotfix support to fix the problem. Before you implement the change in a production environment, we recommend that you test this kind of change thoroughly in a test environment to make sure that no problems exist.