I got an NOS M250r2 with FCO lense as a birthday present. Unfortunately, it contains my least favorite HID source, High Pressure Sodium, and it is 150w. (Why could it not have been 150w PSMH???) Since that would be way to bright for use indoors (I love mercury vapor ), I want to know if it could run 175w Mercury vapor. I know you can disconnect the ignitor, but I haven't even tested it, because I don't have a 150w hps lamp. I have a 70w HPS bucket, and tried a 100w mercury in it, and it just cycled. Would a 175w Mercury vapor or metal halide run on a 150w HPS ballast? Thanks. 

Answer depends on where you are from

In the US - If the HPS is S55 150W - No. S55 is a choke on 120V, Mercury and MH require autotransformer as they require higher voltage. S56 might work to some extent but out of spec and i am not sure how well. (As you mention 175W Mercury i guess you are from the US)

In Europe - Yes, 150W MH works on same ballast as 150W HPS, only swap the socket from E40 to E27. Not all HPS ignitors are rated or MH but most of them work anyway

Yes, I am in the US. I have a 70w PSMH ballast, so I will probably just use that.

The 175W MV may run on S56, but you should deactivate the ignitor (teh HPS needs HV pulses to be ignited, but with MV these pulses may ignite the parasitic discharge outside of the arctube and so destroy the lamp)
Without the ignitor pulses the S56 characteristic pretty matches the MV175W need (OCV above 210V, 1.8A into about 100V arc)...

Yes, I am in the US. I have a 70w PSMH ballast, so I will probably just use that.

Once you finish converting it, post a pic, or email me a pic, and I will give you a contact at GE who may send you a NEMA label sticker.  GE sticker for GE product.  :-D  to  ...  I LIKE IT!!