Sounds like you'll have an easy time with a conversion. You want to bring copper to the outside. If the black plastic has the same outside diameter, you can use one of the existing exit holes. Another exit hole might work to carry wiring from inside to outside. If you don't have any experience soldering copper, you can use the popular alternative of "Sharkbite" compression fittings.
Part of the new work will involve a backflow preventer in the plumbing supply, or as a part of each new zone valve. Is this flat ground, or at least not sloping uphill from the house?
Hmmm sounds like I have to do a lot more research on the conversion. I was hoping i could just put new valves inside and avoid having to put the outside access box etc.
I'll take some photos and measurements... the current plastic lines exit through a field stone foundation, so snaking copper out through the same holes might be tricky, but once I've dug an access pit outside, probably manageable.
So I'm gathering, in broad strokes, The current "manifold" inside consisting of three 3/4" copper stubs would be replaced with a single copper line, with a new backflow installed, leading out to the outside access box. That box would then contain a new manifold and valves leading out to each zone?
Then a new controller, with wires leading back inside from the valves up to that controller.
And as for elevation, that's a good question. I'll have to do some rudimentary surveying. I know at least two zones are solidly downhill. But one is flat, or maybe possibly slightly inclined.