Already did; with a single stage stat the furnace will come on when thermostat calls for heat, stay on low fire unless the furnace continues operation past whatever time the contractor set the controls for, at which time will go into high fire until thermostat is satisfied.
With a two stage stat the furnace will respond to either the first stage of heat by operating on low fire or the second stage of heat by operating on high fire. How much heat is put out is determined by your actual needs.
The benefits of the two stage stat are that if you come home after having set the temperature low while you are out, the furnace will warm up your home quicker. Likewise, if someone has left a door open to bring in groceries or has opened a window because something was overcooked or the dog farted (yea, I know, it's always the dog ), then the thermostat will respond to the rapid drop in temperature and tell the furnace to heat up the home faster.
With the single stage stat the furnace will need to wait out the low fire program before the high fire will kick in to warm up your home quicker under adverse conditions such as I mentioned.