Upstairs bedrooms are too hot. Would like to put in another thermostat.
You can't make your single-zone system behave like a 2-zone system just by changing the thermostat, unfortunately. What you're suggesting would be roughly equivalent to relocating the thermostat upstairs. To do this, you could either move your existing thermostat and wiring, or achieve a similar effect by getting a thermostat with a wireless remote temperature sensor. In the winter, you'd have the thermostat base its operation on the temp downstairs, and in the summer move the sensor upstairs.

But - this would just run the AC more. You'll still have the problem of upstairs being much warmer, even if it's 70 upstairs and 60 downstairs. You could probably achieve a similar effect by just turning down the temperature on your thermostat without moving it.

You mentioned your house was built in 1941. A common cause of poor upstairs cooling in older homes is the lack of a return air duct upstairs. This leaves no escape for hot air that has collected upstairs; the cool air pumped up there is going to settle down first. If you have a return upstairs, try closing your downstairs return in the summer so that your system is forced to pull more from upstairs. If you don't have an upstairs return, try closing some of your downstairs vents, so more of your cool air goes upstairs. Yes, this will make your blower work a little harder, but it's not uncommon and should be fine.

Finally, a low-tech but possibly helpful option would be to add a floor fan blowing air upstairs. Run it for a day or so and see if it makes a difference in equalizing temperatures.