Could be just the TC as mentioned above. But I expect a complete tranny rebuild is what you need, and that will fix whatever’s the problem. When my truck started having trouble shifting in cold weather, it was in the shop for a tranny rebuild soon thereafter, which totally fixed the problem. The shop just takes everything apart and installs everything in the rebuild kit is all. There’s lots of parts to deal with and it has to be done in a very clean environment, making it a time consuming process, that’s why it costs so much.
When I was visiting the shop that rebuilt my truck’s tranny during the process, I was chatting wit the owner when the tech came by with the tranny pan. He showed it to the owner, who looked at it over for about 5 minutes under one of those big magnifier lighting fixtures you see in electronic assembly lines, before pointing out to the tech there were still a few spots needing more cleaning. That’s the level of detail needed for a successful rebuild.
Many newer automatic transmissions are electrically controlled using solenoid valves, so before doing a total rebuild it makes sense to check those, and that their electrical signals are making it to the solenoid. What I think is happening though is that the transmission pump can’t get the pressure high enough anymore, due to slightly wider clearances among the various parts. When that happens the clutches start slipping, and it usually starts showing up in cold weather, especially at the first start of the day.