When you start from the middle of the engine's torque curve............

If you increase diameter..........
The rpm will decrease, the airspeed range will lessen and become more constant, airspeed might decrease slightly both high speed and low speed, idle might improve, prop should pull better through turns, acceleration and deceleration should improve but cover less speed range

If you decrease diameter..........
The opposite of the above

If you increase pitch.........
The rpm will decrease, the top speed should increase, speed at idle should increase, acceleration and deceleration should lessen, the airplane will probably lose speed in turns more

If you decrease pitch...........
The opposite of the above

For all..........
When you increase rpms by whatever means, engine heat should increase as should fuel consumption.
When you decrease, the opposite of the above.