[denizen467]

^^ I'm not sure, but it might be the case that LEDs use DC and consume low voltage. (I'm guessing that the screw-in LED bulbs starting to become available for standard domestic bulb sockets actually have AC adapters built into each bulb -- one thing that would keep them expensive and, if you've held one, very surprisingly heavy.) I presume all the city light poles have AC 120V (or 240V?) running in them. So, as Orulz mentioned above, you would need different fixtures for LEDs. Of course, you theoretically could actually reuse the existing fixtures and have each new LED bulb include all of the necessary conversion stuff, but in the long run it might be better to have low-voltage DC light poles/fixtures and simpler, cheaper LED bulbs.

People above have mentioned how the quality of LED light is different -- I wonder if that means they needed to replace the fixtures' transparent covers too?