The main factor in the conversion is how "smart" the control system is. The older systems with capillary tubes or TX valves are fairly straightforward: disconnecting the air coil and fan is a matter of plumbing/brazing/flaring and a little electrical. The only control factor involved outdoors that is super critical is ensuring the water HX never freezes, ever. This typically only happens in winter with an A/C only unit. With a heat pump, it could happen in heating mode as well. Thus, my recommendation for some extra HX capacity and/or water flow rate.
The newer mini systems (especially variable speed inverter driven) are all over the place as far as system design. Many have electronic expansion valves in them, that are controlled by a PID loop, that gets feedback from some sensor(s). The less complicated controls have 3 thermometers in them. They read indoor ambient/indoor HX temp, outdoor ambient/outdoor HX temp, and compressor discharge temp. So even with these simpler/cheaper rigs, if you change the outdoor HX operating envelope, it could trigger a fault condition. Then the unit would either stop in its tracks or run like crap.
This may work in your favor as far as total project cost. Since the unit will be ground source, low or high ambient temperature operation features can be ignored. With many of the premium rigs, extreme temperature operating efficiency is the reason the unit costs more. Hyper heat, turbo cool, quick chill, etc. modes are the trademark names for these modes where the control shifts gears to achieve different objectives. You can find a less expensive unit without these added functions, and it will perform close to (or better than) the premium unit does in its sweet spot. The less expensive unit won't have half a dozen sensors in it, so the chance of proper modified operation is much higher. Most likely, a good plate HX will be less than the price difference between models of similar capacity.
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