My council are wasting millions replacing street lights, and in most cases, the whole light post, for LED lights. It is going to take 10 years to break even. And to top it off they got a private company doing it which no doubt the top councillors have shares in.
The running cost of a lamp post isn't just the post/bulb and electricity, but the number of times they'll have to be serviced, IIRC LED bulbs should last longer than CFL and other types (how long depends on the bulb type).
So if you only look at the saving in electricity (which is where a lot of the "it'll take years to pay for itself" arguments come in), you only get part of the story, even if it just saves a couple of visits for replacement bulbs over the space of 10 years that is a fairly big chunk of money (mainly in the time of the maintenance crews).
I think one of the reasons they sometimes change the whole post rather than just the bulb is that the lamp posts probably have a recommended design life, or may no longer be made in that style and thus no spares, or the compatible spares are priced such that it's not worth fitting them to a post that is already 10, 20 or 30 years old.
You also have the logistical issues involved - it's probably cheaper to replace a bunch of older model posts and cut the number of different types of spares/designs in use than to keep the parts for them all (especially as newer units can include additional features as standard that had to be retrofitted to older ones).
Our local council replaced a lot of the street lamps around here about 10 years ago, taking down what were obviously older models, and I was slightly surprised when upon reporting one (of the previous generation) posts that was near me for being damaged, the council had a team out within a couple of hours who cut it down (and sealed the stump), then about a week later a team fitted one of the latest models that the council were using. I expected them to simply replace the damaged inspection hatch and repair the wiring.
