Man, this forum doesn't get as much help as the truck side of things.
Anyways, update on my question. IT IS PLUG AND PLAY for my 2021 Yukon XL Denali (build date May 2021).
Several people were saying it was not possible and just to trade up to a 2022 model, but I didn't want to lose any features I currently have. This was by far the best $70 I have spent on this vehicle.
There are 25 part numbers for the switch with camera, so you will need to figure out which is correct for your vehicle based on RPO's. Parts guy at the second dealership I went to (first was not interested in helping me) was willing to print out the list for me to figure out which it was as he could not find it based on a VIN I gave (I have attached the part numbers pages so hopefully others can find their's easier).
Mine was an ultimate:
UHX: Lane departure
JHD: Hill Descent
UD5: parking sensor
KL9: Auto start/stop
UV2: 360 camera.
Z88: GMC
Other RPO's listed:
NSS: no start / stop
X88: Chevy
You can find your RPO code on the driver's side door jamb and scan it with your phone camera to read it.
Once you get the part, it literally took less than 10 minutes.
Popped off the 2 panels: Do the bottom left of the kick panel first, then do the side piece.
remove 7 mm screw that holds the trim piece with the switch.
pull straight out on the trim piece with the switch (4 retainer clips, see picture).
remove clip but first using small flat head to slide red locking piece out (to the right in the picture) and then flip green piece down and it will release the harness.
Remove switch from trim piece with small flathead to release clip at each corner.
Install new switch and then put everything back!