I switched out my front and back porch lights with a 50W High Pressure Sodium fixture (this was in 2003).
Link to the GE bulb that came with the fixture.
A friend has a couple 100W metal halide ED17. here it is.
the metal halide says its a pulse start
the High Pressure Sodium doesn’t list a start method
any chance these are interchangeable? how about the wattage difference?
My buddy has no use for the metal halides and would give them to me.
No they are not. The fixture has a transformer to power the lamps with the appropriate voltage. Metal halide and high pressure sodium operate at different voltages.
ok. well it would be nice.
I could get a metal halide fixture. But they are expensive. Quite a bit more than I paid for my High Pressure Sodium fixture.
not even sure this would work. Its metal halide 100 W not ED17 Geez thats expensive. Even with my trade discount it would be over $160
This is not strictly applicable to the OP’s situation, but there are conversion bulbs for sale that are supposed to allow you to use an HPS bulb in a MH fixture and vice versa - and also switchable ballasts that permit usage of both types of bulbs in one fixture.
Metal halide for vegetative growth, high pressure sodium for flowering. 
I don’t know the ins and outs of these systems but it would likely be best to keep to HPS bulbs in HPS fittings. I have seen a video before of someone using a 250W Metal halide bulb in a Mercury Vapor streetlight, but don’t know about HPS to Metal Halide. And if the worst comes to the worst, you can just sell the metal halide bulbs online anyway. 
US metal halide and HPS lamps are not interchangeable. They have substantially different arc voltages (100 or so vs 55). European HPS lamps, which use a different arc voltage, often are.
A metal halide lamps is basically a mercury lamp with some nasty stuff added that increases the efficiency and color rendering and decreases the life. Older ones (probe start) use a starting electrode like mercury vapor), newer ones (pulse start) use a starter like HPS.
Generally a probe start metal halide is cross compatible with mercury vapor. The current requirements are slightly higher so a metal halide lamps may not work in a mercury ballast, but the reverse is always true. Commonly ballasts were rated for both lamps, now that mercury ballasts are illegal they removed the mercury vapor rating without changing the design.
For growing completely legal herbs in your basement, grow lights often use more expensive electronic ballasts that can accommodate either lamp since switching lamp types is often desirable. This is not true of the cheaper magnetic ballasts for lights on buildings.
Still can’t figure why anyone would use either HPS or metal halide fixtures for outdoor illumination, given the high energy consumption and availability of (for instance) LED fixtures that provide ample light much more cheaply.
hi Theyre not! Better to replace them with of course the like bulbs that you had in your mind when you typed out your post .
As a potential reasons:
Maybe you got a fixture free or cheap on Craigslist
Maybe you want a warm white (ceramic metal halide) or golden yellow color (high pressure sodium). I know warm white LEDs exist but in configurations other than lower lumen A19 retrofits they’re a lot less common than cool white or daylight white LEDs.
LEDs just don’t look right in certain fixtures, like a clear glass globe. If you want something higher than equivalent to about 100 watt incandescent that’s omnidirectional your only option are “corn cob” LEDs that don’ t look as nice as an HID bulb.
I use a 100 watt ceramic metal halide lamp bounced off the ceiling in my office. I wanted a lot of light and I couldn’t find an affordable LED fixture that produced that much. I got the fixture for $5 at Restore; it was originally a 175 watt probe start lamp that I replaced the lamp with a pulse start ceramic and the ballast with an electronic one