Not sure what you are referring to due to nomenclature .
The 'thermal cut out' is the one shot fuse inside the clear tube.
Brown wire from connector on board to the normal t-stat lug.
It can be unplugged and replaced
The normal t-stat and the ECO (high temp t-stat---under the foam cover) can be replaced. They are held tight against tank wall with three tabs which hold a plastic washer and compressed spring down.
Bend the tabs out and remove t-stat(s)
The spade connectors can be cut off and new ones crimped onto the wires.
The normal t-stat uses small spade connectors (brown wires) and the ECO t-stat uses a larger size spade connector (red wires).
Here is a visual.......the label 'flame fuse' is the thermal cut out
