My wife and I own a pair of LR AWD Model 3s that we bought in September and December of 2018. I paid for FSD for both vehicles in January of 2019.
On a whim, I submitted on 03/24 via the mobile app for each vehicle a mobile service request for the FSD computer retrofit, and within a day I received a call to schedule the service at the Santa Clara service center. One was scheduled for 03/26, and the other for 03/27. I then received a call on 03/25 that indicated that the FSD computers won't be available, so the appointments were pushed out one week, to 04/02 and 04/03 at 8AM and 8:30AM, respectively.
I drove one of them (the 2018-12 one) to the service center on 04/02, and my daughter drove behind me in our Mazda CX-5 that she drives to bring me home. I asked whether I could drive the other Model 3 to pick it up later that day, and leave it for its own service the next day. I was told that I could. I was also provided a $200 Uber credit that I didn't need. Although I was not able to access that Model 3 in the mobile app while it was being serviced, I received a notification in the afternoon that its software update was complete. At about 4:30PM I received a text message that indicated that my Model 3 was ready to be picked up. So, I drove the other one to pick it up, then left it for its own service. The software was 2020.12.1 prior to the retrofit, and was still 2020.12.1 after it was done.
For the second Model 3 (the 2018-09 one), I received notification of the software update mid-morning, and then at about 10:30AM I received a text message that it was ready to be picked up. My daughter drove me there. Same deal as the other one, except that I asked for the tires to be rotated ($50). There were also two TSBs that were taken care of: Replace Charge Port Pin Deadfronts & Readjust Hood Latch.
For both vehicles, we lost the profiles, so we had to adjust all of the settings again, to include HomeLink. I was told by my service advisor that there is a 50% chance of retaining the profile settings.
Anyway, both Model 3s are seeing lots of cones. It was a very positive experience overall.