I'm a bit confused on this topic.

Why wouldn't you just replace the in ground casing and plunger? The purpose of the dual post jacks is to eliminate having to drill down but that doesn't seem to be an issue here. Also doesn't the existing elevator have a counterweight? Or is it cantilevered with a sheave?

In my experience dual post elevators are not as reliable, require more maintenance, are more time consuming to install and altogether just aren't as nice a product. I see them as a compromise when you can't drill or otherwise need the space below. I've installed 3 canton single stage roped hole-less and I don't like them and I have now maintain 2 that some hack company that went bust installed and they are a nightmare. I've never installed or seen a telescopic roped hole-less and would be very concerned about how it's stabilized and what maintenance is required. Is it like the inverted 330a jacks? Plastic pulleys and a special repack toolkit? No thank you I say.