I have a gas boiler fhw baseboard system with 2 zones (teco z valves & 1 cir.) and have no heat upstairs. I switched the valve heads between the 2 zones- still the same.
I replaced the t-stat upstairs- still the same. Any suggestions?

Is there a seporate pump that pumps the upstairs and down stairs heat?

Switching valve heads/operators does not prove that the zone valve is good!

Look at the gauge on the boiler and it should be reading 12 to 15 feet psi/altitude.
You water pressure in your boiler could be to low causing the water not to be lifted to the second floor. The pump does not lift the water it only circulates it.

The system has 1 circulator. When I bypass the zone valve with the lever, heat flows.
Either zone valve head/operator works on the downstairs zone.

When I bypass the zone valve with the lever, heat flows.

Then these are the possibilitys

Zone valve not getting power to activate it to turn on the zone.
Trace power back to the source.
Unlikely thermostat bad but check it anyway.