Why cant you switch the lights in the same room? You can still force auto's to flower. Autos will just flower automatically when ready but you can still force autos to flower. If you are saying when your auto is ready to flower you cant switch the light to 12/12 becasue you want to veg your others longer then yes you have to bring it somewhere else. The MH will work just fine but you will not get near the same yeild as if you use an HPS. But mh will still do the job just dont expect the same results