I think it would be a good upgrade for the main lights, as HID lights generally last much longer than Halogen bulbs, and take about half the power. This would result in a very slight reduction in fuel consumption and potential for higher brightness.
The ditch lights, I recommend to stick with Halogen because those lights pulse on and off if the engineer decides to and HID lights are not good with short on/off cycles. LED could be an option for ditch lights, they are getting bright enough now.
EDIT: For the main lights, if it were to be converted to HID, I don't know if the existing fixtures are reflector or projector style, they may have to replace the fixture to properly guide the light.
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