That's actually a good question in this day and age. What panels can replace a 400HPS without making the buyer change his style too much? I think the latest A51 can do that. 2 of Han's latest should. 2 Blackstars 240s are close to an old dirty 400 HPS
. Apache makes a few models that you could drop in. The decent Chinese panel makers like Evergrow can but they still produce a bit of heat and aren't worth it most of the time since your ebill won't be that much lower and your yield could be on the low side at first. Plantphotonics makes a model that should be able to. Cali Light Works has a model or two. Rhino's should. Anyone else? There must be a few I'm missing. Like Lumigrow's
http://www.lumigrow.com/products/pro-series-greenhouse-lights which will probably crush a 400HPS in both power used and yield.
How many plants do you want to grow? What are you pulling off your 400?