My understanding is that it's negligible, and any difference would pay for itself fairly quickly with the savings of heating mostly with the heat pump vs. gas.

As to the OPs question about the "changeover" setpoint to switch to gas heat, I'm not entirely sure but the folks who install the heat pump should absolutely be able to explain how that works. As HokieDiver stated, I think that is typically a programmable setpoint that should automatically kick in at a certain outside temperature. The installing company should determine that setpoint and set everything up.