« The IDA Open Source Migration Guidelines provide practical and detailed recommendations on how to migrate to Open Source Software (OSS)-based office applications, calendaring, e-mail and other standard applications. These guidelines have been designed to help public administrators decide whether a migration to OSS should be undertaken and describe, in broad technical terms, how such a migration could be carried out. They are based on practical experience of a limited number of publicly available case studies, and cover a wide range of management and technical concerns. »
I have not yet had the time to read most of it, but looks promising, full of useful information.