Well, if you documented the calls, as in who you talked to and what they told you, then it seems to me you can complain and maybe they'll reinstate your reservation. However, can you explain what you think they meant by "unless I updated pricing on December 19". Were they telling you that you couldn't get the car at the 2020 price any more?
As a data point, I got the "ultimatum" in November for a Model Y order I made in October, 2019: accept delivery by December 2 or it will be canceled. I had put it off multiple times because I wasn't driving much due to the pandemic. Tesla's always been fine with putting off purchases -- you'll remember they've advocated putting in a reservation in order to lock in FSD pricing before increases. Anyway, I accepted delivery and was given an estimated delivery date of March-April 2022, which suited me fine.
But on Dec. 24 I got the VIN assigned message, with delivery scheduled on Dec. 26-30. That was not what I wanted, but as putting it on hold again would (according to the ultimatum) result in the order being canceled, I took delivery yesterday. The price was $11K less than if I ordered one today, so I can't complain. Paying $6K instead of $10K for FSD was a part of that.
But in the hurried run-up to taking delivery, the VIN was unassigned and then reassigned, delivery times were changed multiple times, estimated delivery was moved back out to March and then pulled in again. Pretty chaotic. And it was clear that nobody there talked to anybody else there.
So if you are convinced they made a mistake, then I suggest you wait until after the end of quarter craziness, give them a week to catch up on sleep, and then see if you can get them to fix their mistake. As to "legal basis" forget it. Tesla will try to be nice and do what they said they would do, if you are polite and pleasant and have reasonable evidence. Best would be to go in to a showroom and interact face to face, as they're always more helpful in person. Best of luck!