So I am looking to replace my heating system with an air source heat pump. My heating system is hydronic and a/c is refrigerant. I am considering converting my current air handler over to hydronic by installing an air to water heat exchanger where the current refrigerant coil is. Has anyone here performed such a swap or can provide some thoughts on the quality of my thinking here? The tight space is what is driving my decision to consider this.

You will likely get less humidity removal with a water-to-air heat exchanger vs. a refrigerant-to-air heat exchanger due to the lower temperature of the typical refrigerant coil and its ability to pull more moisture from the air. Whether or not this is an issue depends on your local environment.

What's the typical temperature for a refrigerant system? I'm looking at 44F from the chiller for dehumidification. I set my thermostat at 75 during the summer and that would mean a dew point of 55F with 50% RH. higher temps and RH means a higher dew point. What sort of things should I consider to make sure I will get enough dehumidification? Would a larger heat exchanger help? How far below the dew point should the temperature in the exchanger be?

Typical residential HVAC system has ~40F at the inlet to the refrigerant evaporator. To improve humidification with a higher temperature coil, you could get a larger heat exchanger or slow down the fan speed. Also insulate the supply line so that it doesn't pick up any measurable heat between the chiller and evaporator.