Switched 400w Mh to Hps Plants a Little Droopy?

kinda new to growing weed indoors any way i switched 400w mh to hps bulb a coup days into 12/12 and my gals lookin a little droopy and spindly they have been very healthy and happy throughout the grow till now mosrtly at the tops lights not to close good vent. too. any advice be much appreciated

The plants look bigger in hps vs mh the only other thing would be more water!