How many miles do you have? The reason I ask is that I did notice that, but I was about 60k miles so I flush, because I was at the mileage not because of the shifts. It did resolve it. You have to remember when the tranny is cold it is at full pressure, and with the fluid not as viscous as it is when its warm, that would give you a harder shift.
It was -22 degrees here this morning so I can confirm the hard shifts. I think it is normal. Often times the transmissions shift differently under extreme cold conditions to help them warm up. I can only imagine how thick the tranny juice is at these temps. By the way, my Titan grunted and groaned when I started her this morning, but fire up she did. Afterwards we went around and jump started 6 cars. I am sure there were many factors on why the other cars and trucks didn't start, but it still makes me feel good.