As I understand the auto tranny, the shifts depend on a combination of car speed and the position of the accelerator pedal. I'm leaving out both vacuum modulation and computer controls which the 240 doesn't use.

When you are accelerating up the hill the tranny will downshift when you either press down on the accelerator or the car speed drops to a shift point. It is normal and necessary for the tranny to downshift in the conditions you describe.

There's usually no need to manually downshift, you can make the downshift happen sooner by pressing farther down on the accelerator. I sometimes manually downshift on a down-hill run when I want to keep a slower speed such as coming downhill into town with a school speed zone at the bottom odf the hill.