There is no way to modify your exsisting furnace, it is what it is. That said, I would keep the gas (replace the furnace) but opt for a heat pump which would give you the best of both worlds.
You already have a 90+ gas. Stay with it since venting is going to be an issue with anything less. Install a new furnace with an ECM motor. You can choose single or two stages, it really is not that big of a deal since it is going to only be used at the lower outdoor temperatures. The ECM motor will add to the efficiency of the heat pump you will be installing in place of that old air conditioning system.
Now, if Comfort is your bag, replace the furnace with a mod90 by Rheem and then install a 16 or 18 SEER, 2 stage a/c for optimum comfort. Granted you dont get the savings of a heat pump, but you'll be the envy of the neighborhood.