I did, not too many days ago, for both the furnace and AC. It was generally straightforward. A furnace only job would be simpler as there are fewer wires to contend with--likely only two, in fact. They would be connected to R and W (or Rh and W if you have Rh and Rc separately on the thermostat). The standard naming of wires and connections for thermostats is rather obtuse.
This thread has my description and a picture (and a handful of other mods I've done sort of recently). Maybe it will be helpful. The Lux thermostat I used is $18 or so on Amazon.