I might have worded wrong. I need to convert my MH ballast to HPS. I'm trying to keep this as cheap as possible.
There is no reason that you cannot use your 400W MH to flower your plants.
Damn straight! Far be it from me to get between a man and a new toy but, people do 100% MH grows all the time.
A regular HPS (not conversion) should yield better than a MH of the same watt rating.