Good responses so far. That said, take a close look at the rubber tube between the air filter box and the intake. We bought our 2014 Impala used and it had about 28k miles on it. The shifting was a bit wonky, especially in situations when I expected it to downshift. Then I if found a large crack in that rubber tube and replaced it. That addressed all of my shifting concerns. The problem is that the crack was allowing unmetered air into the engine; so the PCM wasn't seeing as much of a load signal as it should. (The mass air flow sensor is on the air filter box side of that tube. So any air coming through the crack was not being seen by the MAF sensor.) It's a quick and easy replacement if that's your problem.