When You changed the fluid, which Dexron fluid did you use?- hopefully Dex6 ??
Normally a P1811 code will show up, which scanner did you use?
Most of the time these transmissions experience hard shifts after being hot a while.
You may have varnish building up on solenoids or inside accumulator, so before you drop the pan and replace it with Dex6, add a half to a full can (depending how much room you got to just above full line when hot on dipstick)
Add SeaFoam Trans Tune.
Trans Tune - Sea Foam Sales Company
You may notice an improvement quickly, drive with it in for about 750 miles, after adding it, then change fluid again.