When Subversion updates a WC, it normally expects to transfer just the changes. Since the server in this case doesn't have a copy of the old version, we need to modify the 'update' mechanism to cope with that. We also need to consider how the behaviour of this kind of update (a nonexistent version to/from an existing version) relates to the behaviour you get if you ask to update a nonexistent version of a normal Subversion-controlled file to/from an existing version of it. There are important differences.