Main differences:
-Transmission went from 8-gear to 10-gear
-Some older models do not have AndroidAuto and/or Apple CarPlay
-Newer models got HDMI port for the rear entertainment system, while older was RCA connection port and SD card
-Newer model got a new amazing updated grill
-I know that newer model got an Active Grill Shutter as standard, but I think it might not be installed at the older models
-Older models might not have the large digital display in the middle of the speedometer
-Earlier models got some recalls that you might need to check, last one was in Sept 2019 for the break vacuum pump and I guess 2018 was not included since they already have fixed it
In general, you may need to check Fourms and YouTube regarding transmission thermostat delete to keep transmission fluid cooler
Active Grill Shutters were standard for model '17. Not installed on earlier models.
Large digital display is available on earlier models, just select it via the menu.
Once I had my 8 speed trans reprogrammed, the issues were about 90% solved. No more hunting for gears, and most upshifts are very smooth. Only occasionally do I feel a hard upshift. No one but me seems to notice.
Updated grill fits older models perfectly, and is relatively easy to install. Didn't cost very much either. I also changed my '16 to '19 wheels.
Model '17 dash lighting in the Yukon's went to blue, instead of red. Cost cutting move perhaps, as Tahoe has blue lighting too.