Like all the lightbulbs blowing in your house at the same time, they have a lifecycle and since a decent number of them were installed in the same timeframe, th

Like all the lightbulbs blowing in your house at the same time, they have a lifecycle and since a decent number of them were installed in the same timeframe, they're probably coming due a replacement at the same time ("same time" being the split across the 7 years referenced). Since you're planning on having a multi-year infra project on the go and getting the expertise rolling, it's probably worth extending it across the network (even the newer ones), as I'd assume there's some economies of scale involved in parts and labour. Judging by other comments in the thread, they're reaching their natural life.