I just traded in my 2017 Model S P100D for a 2025 Air Touring. The process is pretty simple: the SA sends you a link to kbb.com to fill out and upload docs & pictures, and you get a preliminary value. The Lucid KBB rep then sets up a video (Teams) call to do a virtual walkthrough, after which your estimated value decreases a bit (typical, according to my SA). All told, it was just 3 days for the whole process and would have been faster if not for the big end-of-quarter backlog. I was not pleased with the value they came up with, especially given the fantastic shape my car was in, FSD upgrade, and free supercharging, but Tesla values are pretty depressed right now so I guess it's to be expected. However, the $2K conquest bonus (discount) and $2K Tesla trade-in credit helped make up for the difference. With all the Lucid Q1 incentives, it was impossible to argue with the final lease monthly payment, which was phenomenal.
WRT the GT with 20/21 wheels, this is what I spec'ed on my GGT. The 450 mi range is compelling. While I am OK with the wheel size, I'm not as sure about the rubber. You might watch this
Tire Rack test, which did not rate the Hankook Ion EVO tires very highly. They were fine in the dry, but did not handle well in the wet. I am currently deciding if I will proceed with the 20/21 wheels and then swap out the tires, or go with the 21/22s with Michelin Primacy AS rubber. That's a good-but-not-great tire, so still deciding. I do like the wheel design better. I will probably decide after my test drive.
Bottom line, I thought the KBB and overall trade-in process went well, and Lucid handled it all expediently and professionally.