Update mode in setup:

Autoreponse: UPDATEMODE=IF_AVAILABLE

setup2/source/other/respfile.cxx (ReadUpdateMode)
    eUpdate = IF_AVAILABLE ...

(ProcessEvents)
    if (bIsUpdate)
        bDoUpdate = TRUE;

            if ( bDoUpdate )
            {
                if ( pEnv->IsUpdateOldVersionNoRights() )
                    Critical_Error( ERR_UPDATE_NO_RIGHTS, "" );
                if ( pEnv->IsUpdateOldVersionWrongLanguage() )
                    Critical_Error( ERR_UPDATE_WRONG_LANGUAGE, "" );
                
                pEnv->SetUpdateOldVersion( TRUE );
                nReturnEvent = EVT_START_UPDATE_OLD_VERSION;
                break;
            }

-> setup2/source/ui/mainevt.cxx (UserEvent)
   -> (RunUpdateOldVersion)