LED retrofits for parking lot and site lighting fixtures are being done all the time nowadays. If you factor in the slow lumen depreciation, which is typically 70 percent lumen output at 50,000 hours, reduced input wattage, and reduced maintenance costs (No need to re-lamp every 10,000 to 15,000 hours as you would do for a metal halide fixture), and higher quality of light (No color shifting), it's a no-brainer.
Some things to consider:
1. From an illumination performance standpoint, a one-to-one replacement should be feasible in most cases. Well-designed LED lighting fixtures are designed with optical systems that can widely and evenly distribute light. You won't observe "hot spots" as you would with metal halide fixtures, but the illumination should be more uniform. Try to find out what the IES distribution patterns are on your existing fixtures and ensure that the new ones match. On a similar note, ensure that a manufacturer is specifying whether a fixture's lumen package is the *overall* lumen output or the *delivered* lumen output. A 4,000 lumen fixture could deliver more lumens than a poorly-designed 4,500 lumen fixture.
2. When the time comes to eventually replace an LED fixture, different manufacturers offer different solutions. Some are making the claim that they'll manufacture and sell replacement LED assemblies from now to an eternity - something that I sincerely doubt. Others are expecting the fixture to be replaced in its entirety.
3. Go with a reputable manufacturer that publishes LM-79, LM-80, and maybe even TM-21 (This is a newly released standard) data on their LEDs. Some lighting fixture manufacturers are making highly unrealistic and exaggerated claims regarding the lumen depreciation and estimated useful life of their LEDs. The worst one I've seen to this date was from a manufacturer that was claiming 70 percent lumen output after somewhere between 150,000 and 200,000 hours of operation.
4. Avoid LED retrofit lamps for metal halide (And fluorescent, for that matter) fixtures. LEDs are unidirectional light sources and perform poorly when retrofitted in a fixture whose optical system was designed for an omnidirectional light source, such as a metal halide or fluorescent lamp.