Requirements for Changing an Object Version
An object version is unchangeable if it is frozen or if it is checked out to a workspace and the attempt to change it does not occur within the context of that workspace.
Note This restriction applies when the repository engine automatically attempts to change an object version for you. The repository engine can automatically change an object during delete propagation. This occurs when a propagated deletion of a destination object version reduces the TargetVersions collection of a corresponding origin object version's origin relationship. In effect, the origin object version has been modified automatically by the repository engine.
This restriction also applies when you attempt explicitly to modify an object, for example, by setting one of its properties.