Maybe it's possible in older models. I have a 2021 Model X and asked a service center if I could downgrade the software after the new UI came out, and they told me it was impossible and that not even they could to it.
It's possible they were lying, I suppose, it would not be the first time, but it seems an odd thing to lie about.
Having XOO,OOO to XX,000,000 cars globally on N different firmware packages when your approach involves fast iteration and OTA updates, in an industry where you catch partial flak for any incident regardless of the party at fault ....
Seems reasonable to not allow random downgrading because you didn't like the UI layout.
Well, I can understand the decision, even if I don't agree with it. The new UI has some quite serious (and now, well publicized) design flaws that could lead to a safety issue.
The correct solution would be for Tesla to fix the design flaws, of course. Or maybe to actually test their own products to find such obvious problems before they are released.
It's possible they were lying, I suppose, it would not be the first time, but it seems an odd thing to lie about.