Today I noticed that the LCD display was blank. I replaced the batteries. I don't know how long it was blank, but it was long enough to lose its schedule programming, including the Celsius setting.
I scoured the internet and found a manual for a different Sears thermostat model. It said to press and hold the two TIME buttons at the same time to switch between Fahrenheit and Celsius. Ours doesn't have those two buttons.
I found many requests from others looking for the manual for this particular model, and a few others. What I didn't find was a solution to my problem.
I walked back over to the thermostat and tried resetting it again by removing the batteries. Maybe I missed something when it first powers up. Nope.
I started pressing buttons in combination with each other. After a few minutes I saw a single number on the display instead of the usual time and temp. It said 78. I pressed buttons again, this time the single number was 26.
If you have one of these thermostats, you need to press and hold the HOLD TEMP button and then press the TIME button to toggle it between Fahrenheit and Celsius.
You're welcome. :)