It’s an interesting subject and when I’ve spoken with other engineers in the past we have had differing opinions / results. However, I think you need to consider all of the costs and our simple calculations showed a payback period in energy costs alone of around 8 years. So on the basis of a 12 year lifetime you are getting an extra 4 years of energy savings, as well as bulk change (once a year maybe?) and occasional lamp faults. And you help to meet the Government target in respect of energy use/carbon footprint so the benefits to the Authority are greater than just the immediate signal section benefits. Having said all of that, we no longer carry out retrofit of aspects due to all of the problems we have had and only fit LED aspects to new and refurbished sites.
In respect of replacement, for the past 11 years our contract has stipulated that the maintenance contractor meets the costs of replacement with the Authority meeting the TM costs. On tall poles this usually means mobile works using an IPV and MEWP so we wait until there are several failures before tackling them all in one sunday morning operation.