Folks-  

At one of my church buildings, we have 2 400W MH lamps that light up a steeple up top.  They have now gone EoL and are dead.  I climbed up there last night expecting to find what I have found at all the other buildings I work on, either a 175W Metal Halide bulb, or a 100 or 150W HPS bulb in each fixture.  I was somewhat surprised to find the fixture to be set up for a 400W MH. I figured since it was a MH it had probably been burned out since after a year or so when the building was built back in the late 90s but it does appear that the bulbs have been changed once.  I got back down knowing I did not have anything that would work in that fixture and with that base type, save for a few 150W HPS bulbs we use in parking lot fixtures.  

Now my question at this point would be I believe that I have a few 250 Watt HPS and maybe 1 or 2 HPS 400 watt bulbs in the "Light Shed" and also a 175 Watt MH with that same larger base type that I had ordered on accident once, wanting the standard medium base but not looking close enough.  Anyway, Could I put in a lower OR equivalent wattage HPS bulb in place of a MH bulb?  Or will the ballasts need to be changed out?  I am asking because first off, 400 watts is way too much electricity to be wasting to light a Steeple where 100 and 150W HPS and 175W MH light the steeples of the other buildings I work on and do just fine.  Secondly, I don't want to be paying $44.00 a bulb.   Now I am somewhat pleased to see that these 400W MH lamps have a 20,000 hour life expectancy and wish that the medium based 175 and 150W MH bulbs would last that long!    

Eventually I am thinking about converting the steeple lights at all the buildings save for a handful over to LED as we want to stay off the roofs as much as possible due to the ceramic tiles cracking very easily when walking on them. 

Your thoughts?

HPS lamps only work with HPS ballasts. You need a 400 Watt metal halide (M59) or a 400 Watt mercury vapor (H33) lamp. I would recommend a mercury vapor since they last longer (24,000+ hours) and are cheaper. The /DX phosphor would probably look better since clear mercury vapor lamps produce very blue/green light. Eye (Iwasaki) lamps should perform well, or any lamp made in Hungary or the U.S.A.

Otherwise, you need to change the ballast to run a different type or different wattage lamp. 70 or 100 watt HPS should be bright enough. Stay away from the HPS bulbs that are made in China; they start cycling after only a couple thousand hours, and need to be replaced.

I wouldn't even consider LED - one lightning strike could fry the circuit board and you'll be be back on the roof replacing the fixture.

I had to ask because I am able to run 100 and 150 Watt HPS bulbs in some MH ballasted fixtures (not all) which is why I was wondering if the same would work for something with the 400 Watters... I know that HPS ballasted fixtures won't run a MH though. 

As I understood, the question came from the "120V" part of the world, so the following is related to that area:
It worked, because at first both 100 and 150W MH are of a pulse start type, so deliver the HV pulse required to start the HPS.
The 100W HPS is of lower arc voltage than 100W MH, so although it run most likely below the rated 100W, it may have operated somehow. But oit mean the MH ballast was feeding a load with lower arc voltage than it was designed for, so was most likely overheating a bit. So even when it seems to work, not a good idea at all.

A bit other situation is with the S56 HPS: The only basic parameter difference from 150W MH is the MH requires higher voltage pulse from the ignitor, otherwise the nominal arc voltages and currents are the same. Only the load characteristic of the HPS may differ, but there the MH is stricter.
So if the socket and installation are really rated for the higher pulse voltage, it should have no issue with the S56 HPS.

But the 400W MH you spoke about is most likely the probe start type, so that at first does not feature any high voltage pulser (so the HPS won't ignite) and if so, it will most likely extinguish after it warms up a bit, because a 400W HPS has too high arc voltage for the given ballast.

And operating lower wattage lamp in a higher wattage fixture you should "forget" about. Remember, it is the ballast and not the lamp, what (in the first order approach) dictate the power delivered to the lamp. So if you put a 250W lamp into a 400W fixture (assume MV or probe MH at this moment), the lamp will be fed by the 400W. And that would be way too much for a just 250W arctube...

Ok thank you this makes sense to me now.  I once saw what happened to the ballast and bulb when I put a 35W HPS bulb in a fixture that was set up for a 70 or 100W HPS.. blew both the bulb and ballast  

MH bulbs on HPS ballast--You can get "Conversion" bulbs-MH bulbs that can be run on HPS ballasts.They are in 250W and 400W GE,Venture makes them.Philips makes an EXCELLENT Halide conversion bulb-Mastercolor-RetroWhite.Use these and they give excellent life and consistent color.They are a ceramic MH bulb.Numerous websites sell these lamps.Try them on your HPS fixtures and you won't go back!I have used the Venture conversion bulbs,too.These work well but not as good as the Philips.

I would either keep using 400-watt MH or MV lamps or change the ballasts. And you definitely shouldn't be paying $44 for a 400W MH lamp. If you are, you're getting ripped off big-time. $15-20 is reasonable for a name-brand lamp, and generic ones can be had for less than $10.