I'd advise against trying. While the bulb might fit in the socket, the igniter, capacitor, and ballast are all sized to the bulb. Having the wrong bulb in is likely to result in a very short life for the ballast.
While you're changing ballasts- why not upgrade to Metal Halide? MH bulbs do not decrese in brightness as they age, as mercury bulbs do, yet still give a clean white light.
About 3 years ago I relamped about 10 70W med. base HPS lamps. I was out at the same place a couple of months ago and I had to replace a ballast. Much to my suprise the ballast was 100W. Lamp and ballast life seem to be uneffected by this mismatch. But I would not reccomend it.
[This message has been edited by Old Town (edited 01-18-2006).]
MH bulbs do not decrese in brightness as they age, as mercury bulbs do, yet still give a clean white light.
That's not true at all. All HID lighting sources have declining lumen output as they age. HPS has the best lumem maintenance, followed by MH and then MV.
The basic probe start MH lamps have poor lumen maintenance, especially as they near the end of their rated life. However, pulse start and the newer ceramic metal halide systems have pretty well conquered this problem.
The voltage referrs only to the fixture's input voltage, not the voltage seen by the lamp. IIRC merc vapors operate at around 80 volts once the arc is struck.
And the 250 watt bulb in the 400 watt fixture is an extremely bad idea. If you're lucky all that will happen is the lamp won't light. Otherwise you could easily have bulbs exploding or ballasts going down in flames.
Either get the right bulbs or get rebuild kits that will have the bulb, ballast and any other essentials.
If you are going to change everything you might as well go to HPS if colour is not an issue.. I believe the "lumens per watt" is greater for HPS than metal halide or old school mercury... Also, double check your input voltage.. I once put a ballast in a fixture that was labelled " 230V". and had a " 115 / 230V" dual rated ballast.. too bad for me the input WAS 600V! it looked spectacular for about 6 seconds when everything was turned on!