just curious how much you've paid someone to replace a thermostat?

$100 total for my A/C guy to install two Nest thermostats - upstairs and downstairs.

Zero dollars. Me or my husband replace them ourselves. They are super simple.

We have an early 90s home with 2 units. The thermostats themselves seemed to be a late 80s model with mercury, cooling unit, heating unit, and an auxiliary heating unit.

I bought two Ecobee thermostats, opened them up, attempted to do it myself, then got to the extra wires for the auxiliary unit and it threw me off. Didn't know if they needed the extra power converter that is included in the Ecobee or not. Even called the Ecobee support line for guidance and they didn't even know.

Ended up paying an AC guy $150 for both thermostat installation. Worth it for me to know it was done right.

what about to move one

I have done half dozen or so of these at houses we have lived in. Most common issue in the power to the tstat. Older programmables used a batter to keep day of week, time and run schedule.

The new smart thermostats need power from the AC unit via the C wire. If no C wire then you get into the adapters. But before you do that, see if you have an unused wire in the wire bundle to connect C to C (as per instructions, my comments are not instructions but things to look in to).

Regarding the question on moving a tstat, that not merely a matter of funning the wires. Placement of the tstat is not arbitrary and can adversely impact the HVAC efficiency.