From DeveloperWorks, Versioning business processes and human tasks in WebSphere Process Server
This article shows you how to build versions of business processes and human tasks that are based on best practices in IBM WebSphere Integration Developer V6.1 and WebSphere Process Server V6.1.
Versioning of a Business Process is the most important feature of the BPEL Container, and likely the feature you know the least about. The choices you make in how to invoke parent-child processes define the way they behave when the BPEL templates are updated. If you wire them together in the assembly editor, then the two specific versions of the template are bound together forever. Parent process A v1.0 will always use Child Process B v1.0. If child B 2.0 is deployed, A will continue to use 1.0. Not exactly the behavior you would expect.
This article defines the way that the BPEL engine resolves versions and it should be required reading.