As others have said, I booked my MCU1 to MCU2 upgrade in early December, but within just a couple days contacted and indicated must be done at service center. My understanding for that is they need to reload a lot of firmware, pull setting back down from cloud, and likely want (need?) car connected to service wifi to accomplish this.
As for how long it takes, it actually only takes 2-3 hours. Comment from my service advisor is most of that time is actually for reloading software/downloading settings from cloud, etc. Are some number of cases where that may not work well first time and take a little longer (my service advisor indicated maybe 1 in 10?). In my case is was in and out in 3 hours. Only thing I needed to reset was reprogramming my homelink. It showed my devices were still there, but had to go through the reprogramming with remote next day to get them to work.
Those that are experiencing much longer, or multiple days, to get this repeated, then the issue is that your service center is likely scheduling more appointments than they have labor/space to support, so you're car is sitting waiting in the queue for most of that time.