Believe we are coming to the end of this long journey.
When it first appeared, they attended to it by replacing the units, under warranty.
After perhaps thousand of these, at over $1,000 each they began to get reports back that replacing them often did no good. the replaced ones just yellowed again. Customers did not want to keep taking their cars back in, and the corporate bean counters were not happy to be spending all that $ with no good results.
They evidently tried the software route. Changing the display brightness, tone and colors. That did not satisfy anyone, inside or outside the company.
They came out with new part numbered displays, but alas they yellowed as well. Customers were getting irate, clogging up service centers, going to arbitration, complaining on the forums. Nobody was happy about all this.
Tesla realized that something had to be done, but so far every effort had failed. Service center and other employees were sending out conficting stories. Some forum posters even claimed to have insider information and that also led to no resolution.
Tesla went to their strength...technology. They put good people on the task, and evidently they came up with some sort of UV or other spectrum light treatment. They did some in house treatments and evidently got the results they (and their customers) were looking for. They built some of the gadgets and began taking it around to teach the service centers how to use it. Seems like it uses lots of power, specialized spectrum light, cooling fans and needs to be engaged for a certain period of time.
Of course, time will tell if this is the solution that will make everyone happy. They are doing onsies/twosies of different ages and different amounts of yellowing. Will develop a protocol to begin getting all the yellow banded screens clear again.
They have agreed that this is a warranty item. They most likely will continue to treat customer screens, well after their warranties expire, as good will and doing the right thing. They have not announced anything publicly, as are still getting a handle on how to best use the gadget.
They are asking patience from their long suffering customers, until they can get the data feedback on the best way to proceed.
The only thing for certain is that replacing one bad screen with another bad screen is not the solution anyone is looking for. (Insert Star Trek music theme here
Believe the end result will be some sort of intense light bleaching.