When 9.2 SP1 is installed it overwrites pre-existing registry entries that allow for single-use and concurrent-use license to be used and to switch between them.
Either run the install again and this will make certain necessary registry changes or you can manually change the registry to include these lines in HKEY_LOCAL_MACHINE\SOFTWARE\ESRI\License: ArcGIS_Installed TRUESingleUse_Installed Viewer (or which ever program they're using as sinlge-use) If either of these is missing or has a different value, simply change/add them so that you have both. Then you should be able to load Desktop Administrator and switch between them.