Living in a 2.0 world: Versioning our Standards

We are beginning to approach a phase where we need to support a 2nd generation of standards while the first generation still exists (e.g. VOTable, 2nd generation DAL, v1.1 of VOResource extensions). We recognize that we cannot "upgrade" the entire IVOA simultaneously, so we must live with both generations existing in the system simultaneously. Are our applications prepared for this? How should applications react when a version of a service or document is not supported? Are their any issues that will complicate life for users?