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 Repository automatically attempts to change an object version for you. Repository can automatically change an object during delete propagation, when a propagated deletion of a destination object version can reduce the TargetVersions collection of a corresponding origin object version’s origin relationship. In effect, the origin object version has been modified automatically by Repository.
This restriction also applies when you attempt explicitly to modify an object — by setting one of its properties, for example.