I have (electric) central heat and air throughout the house with a separate unit for upstairs. The units are 12 yrs old. Both thermostats are older (mercury readings, I think). The upstairs thermostat doesn't appear to record the proper temperature but will kick on and off according to the thermostat. The downstairs fan blows continuously, even when the thermostat switch is set to "off". Do I need new thermostats and if so, what type thermostats are recommended. I can call an HVAC specialist if needed but feel my electrician hubby is well able to switch out a thermostat. Thanks.

Switching tstats is well within the realm of DIYers. We get many posts here on that subject. Heat Pumps with dual fuel setups tend to trip up most people, but if as you say you don't have that it should be pretty easy.

Honeywell seems to be very popular and there are many posts and a fair amount of support they will give.

What brand tstat do you have now?