dbunke

On my 2015 2500HD 6 spd trans if you have the manual mode in 2nd gear at a stop it will take off in 2nd gear. Not sure what the newer 8 and 10 speeds do but my 6L90 trans will down shift automatically as needed in manual but will not upshift without being told to in Manual mode.

You said you changed the oil a couple days ago, did you see or look for glitter in it? A chewed up cam and lifter usually leaves something in the oil. No glitter, I would open it up. 15 hrs seems high just for tear down..... 175K, there is plenty of life left in it.

On the LS motors lifter replacement does involve removing the Cylinder heads so it is a bigger job than most engines. No hard to do just more involved and costly at any shop.

I just did the same thing with my 2015 2500hd 6L90 trans. With the 2500 you don't have to worry about the exhaust pipe though. I welded a bung to the out side of the pan and used a short drain plug so it does not protrude into the pan at all. I also used a new GM pan bought at Rock Auto for $13..... I will now change fluid every year..... Another bonus of the 2500 is there is no Trans cooler thermostat. Even in summer temps pulling a 10,000 lb trailer I have never seen over 180 degree fluid. Mostly 150 - 160 out on the highway. At 40,000 miles my fluid was still red and smelled ok.