I'm asking for a friend who I will refer here, but I wanted to get the ball rolling.

They have a geothermal unit about 12 years old in a 2 story house. The upstairs thermostat has stopped registering the room temp. When they switch the 1st and 2nd floor stats, the upstairs will register the room temp using the stat from the first floor, but that upstairs stat will not work when it is put downstairs. They are pulling the front part off and switching them so they are not switching wires.  The stat is apparently the problem. They did change the batteries in it.

They purchased another brand (not specifically waterfurnace brand) that they said was the same type, but it did not work.

The waterfurnace brand is over $400 online from a dealer.

They have a service call set up for a $100 dispatch fee and I'm just trying to figure out if this is something they should be able to use a different brand stat on before the dealer sticks them with a bill for $500. Would any of the smart thermostats work such as Nest or Honeywell?

Could there be anything more sinister wrong than just a bad thermostat?

Any thoughts are appreciated.