How can I use a pre-installed A/C unit for heat?
In theory, yes, you could repurpose an A/C unit to be a heat pump but that's not a good choice.  In order to switch between A/C mode and heating mode, you would have to re-plumb the lines every time you switched between heating and cooling or fashion some sort of reversing valve mechanism of your own design to accommodate this.  The American Standard 4A7A6048J1000AA is NOT going to have a heating mode since this installation also has a gas furnace and the extra cost to put in a heat pump where it's not needed is just wasted money.  Retrofitting is going to be a complete custom job and you're pretty much on your own doing something like this.

Not only that but it's going to be extraordinarily inefficient since the unit is designed as an A/C not as a reversible heat pump system.

The best choice, in my opinion, is to use your installation as designed, the A/C unit for cooling and the gas powered furnace for heating.  Gas heating, especially in TX, is going to be significantly less expensive for heating and electric powered heating.

If you are set on eliminating the need for gas, the best approach is to replace the A/C unit with a heat pump system that would efficiently perform both heating and cooling.