I tried to see if I could find similar info, but definitely threadzilla isn't the place to go for it. I reviewed the EPA docs in alot of depth. Fixes aren't approved, but I'm very confident that the "fix" has already been created. Approval is subject to the durability testing.
If you read through the documents, it talks about GEN3 vehicles requiring an additional NOx sensor to be installed. I would infer from the documents that there is no downstream NOx sensor to measure the efficiency of the DEF-NOx reaction. DEF dosing is probably based on a value from the upstream NOx sensor (there is one before the Urea injector). But without a way to measure the NOx at the other end, and thus the NOx -> N2 conversion efficiency, the emissions system can't do a great job of controlling NOx. Since manufacturers want to reduce ammonia slip (injecting too much DEF allowing excess ammonia to be present in the exhaust), the DEF dosing strategy probably allowed high NOx emissions to occur under heavy loading, since there is no way to adjust DEF dosage based upon realtime NOx emissions. I'm sure not enough DEF was being injected into the exhaust stream.
The software reprogramming will probably change the dosing strategy and incorporate the second sensor. I believe this would be the first part of the recall. The second part of the recall would be dependent on durability testing. Generally, diesel oxidation catalysts (DOC) used in diesel vehicles with SCR play a part in converting NO to NO2, as a higher NO2:NO ratio will result in a greater NOx + NH3(ammonia) -> N2 conversion when you get to the SCR catalyst. The DPF in our vehicles is coated with an SCR catalyst. Don't want to get into the reactions, but the point is that the DOC and/or SCR catalyst will likely undergo more stress with the new DEF dosing strategy than it was originally designed for. After the results of the durability testing, they will know how many miles the catalytic converter is good for. If it doesn't hit 150,000 miles, the EPA will require VW to replace the combined DOC/DPF/SCR unit at a specified interval. That will be the second part of the recall, because it will be required to keep the vehicles emissions compliant for the full useful life of the vehicle, which is specified as 150,000 miles for the emissions bin that the vehicles were certified to.
Granted, this is speculation, but it is based on sound science and the EPA's consent decree document. I couldn't help but dive into all this since my background is in Chemical Engineering.