Well we don't get much weather at 15-20 degrees, but I see no normal reason for harsher shifts when cold.
The likely reason is lack of power from the engine and/or vacuum leaks. The transmission responds to manifold vacuum and throttle position. Anything that reduces mnifold vacuum causes harsh shifting. In other words if you have to push the accelerator further to get the same power cold as hot then you will shift under much higher modulator pressure. Be sure the engine is performing correctly, drop the modulator pressure by a turn and make the thing shift earlier by lengthening the control pressure cable. Season to suit.
Bosch Master, ASE Master, L1
33 years MB technician