Less work and means there will also be one lamp working. The labour cost of swapping lanterns between columns is greater than the cost of lanterns.What do you suppose will happen with the LEDs on the old columns, disposed of prematurely or kept in storage as spare stock for situations such as replacing columns after car crashes etc?
I would like to think that redundant LED lanterns (especially if they are of the LA's 'standard stock' and in really good condition), would be put back into store for 'ad hoc' replacements. It was always the practice where I live, especially when SOX ruled the roost, with plenty of Beta Fives and GR150s kicking around in the depot ready to be reused, especially in areas with problems of vandalism.
Although my local authority has made great steps towards the elimination of SON from its roads in favour of LED, a few roads still retain it. It has been the practice to reused redundant lanterns to replace any failures, either to improve the lighting in an area which will be re-lit at some point in the future, or to avoid a 'mishmash' of lightsources.
That said, in order to salvage what could be practically new lanterns, you do need to have somewhere to hold onto them. Rationalisation of highways depots and stores, has meant that there just isn't the space to store much salvaged stuff anymore. Unless it is stored carefully (and logged on a database), it tends to get damaged, lost or just forgotten about. Subsequent changes in ordering policy or manufacturer, will mean you have a depot full of slightly used, but 'non standard' inventory.
Finally, the economics tend not to favour reuse, (especially in the public sector, I presume) as any savings achieved by the altruistic reuse of equipment, is 'rewarded' with a reduced budget the following year. This is why council departments, have traditionally made sure any surplus cash is spent up before the year ends.
In the event of a column being knocked down, the full cost of replacement (including the transfer of the cable etc), is unlikely to be reduced by much at all, just because you have reused an old lantern. As has been mentioned previously, the labour cost has a major bearing too. Presumably most knockdowns are paid for via insurance policies, therefore even less reason to save a few bob.
Even if you do reuse a lantern, its full service history will be a mystery and it will presumably be out of its warranty period too. The cost of repairing it (if it fails soon after), will probably be higher than ordering a new one in the first place.
Perhaps this is why so many LED lanterns are now appearing on the online market places, or even being sent for recycling.