After having delivered SOA Training to various clients this summer and fall many have asked how to deal with change. The contract is the center piece of services and with an effective versioning strategy it can be difficult to alter due to coupling that occurs with service consumers. David Orchard, James Pasley and others have been documenting versioning strategies. From my work with David on the SOA Patterns book(Erl 2008) and my consulting with organizations employing Agile Methodology, contract refactoring/change is a mandatory requirement.
Identifying a versioning strategy such as Strict, Flexible or Loose along with Versioning Identification and Compatibility design patterns provides a foundation for indicating change and providing consistency in managing the consumer impact.
For further details check out a presentation related to mixing Agile along with SOA Design Patterns and Meet in the Middle Strategy.