My hunch based really on nothing: (executive summary: skip to #7 for my guess of timeframe of a possible for-fee MCU2 upgrade option)
1. Once the FSD AP3/HW3 upgrades to all S/X/3 cars with MCU2 are complete, they will be done with any FSD HW upgrades for M3 and all for S/X that have MCU2. Guessing sometime in Q1 or early Q2
2. Completion of #1 is a huge win and gets a big piece of HW upgrades completed and behind them, and has to take precedence.
3. Now we will see how/when they address the AP upgrades for FSD S/X vehicles with MCU1. No reason to believe that this will require an upgrade of MCU1, but I suppose that is remotely possible. This will be the first time they have to make the software deal with the combination of AP3 and MCU1, and another reason why I don’t think it will begin until #1 is complete, just another step that doesn’t have to be done first.
4. When they start upgrading FSD S/X cars to AP3 that have MCU1, I think they will want to do those first that pose the fewest challenges in software or hardware. That leads me to suspect they will start with those having AP2.5. I think this is a smaller subset, AP2.5/MCU1 combination was around for a much shorter time than AP2.0, and also newer hardware assemblies so the least amount of differences. My car was manufactured only a week or two before the MCU2 cutover.
5. Once we see one FSD vehicle with MCU1 upgraded to AP3, we will know whether there are any issues with MCU1. If there are, and if they are force to upgrade MCU1 to MCU2 as part of the process, they may be forced to offer a priced option to upgrade MCU1 to MCU2, independent from any FSD upgrade concern. As I said, I think that is unlikely, that my car will get upgraded to AP3/HW3 and retain its MCU1.
6. In one order or the other, once all FSD S/X MCU1 vehicles with either AP2.0 or AP2.5 have been upgraded to AP3, still retaining their MCU1, ONLY THEN will Tesla consider offering an MCU2 HW upgrade option for a fee. They simply cannot afford the workload until they get all the FSD upgrades out of the way. Totally wild guess for completion of all FSD upgrades to all cars including AP2.0 and AP2.5: by end of 2020, since lots more FSD features will be out by then, and the clamor for AP3 upgrades from those with pre-2018 FSD cars will be huge.
7. SO, only after all of the above, which might be at the end of 2020, will there IMHO be any likelihood of an MCU1-MCU2 upgrade option at owner cost. UNLESS it gets necessitated by fulfilling the FSD option for these older cars, which I think is unlikely.
As I said, above is based on pure conjecture but there is a semblance of logic to it.