My dealer called and told me. No letter from Alta.
From speaking to Marc some weeks ago I don't think it's the MXR upgrade. He mentioned yes, harnesses are involved. However, there were several involved. Harness to battery, harness to inverter, etc. The fix wasn't reverse compatible (in their minds) as some interface connectors were changed and some terminations went from pinned to solder joints to reduce resistance. The entire electrical system was impacted to some extent and the amount of work to incorporate or even make "as good as" would have been prohibitive.
No, for something like this it must be safety or warranty (reputation) related. Like one of their test bikes almost burned down a garage while charging or they got a bad batch of cells that degrade too quickly (I'm speculating). Of course, one can create the other. A charging schedule that's too aggressive can lead to overloading supply power circuits and cause cells to degrade faster than normal. Once the cells are damaged the only corrective action is to replace them. Better to do it now than to receive customer criticism a few years down the line about how their battery pack barely made 500 cycles before it went kaput! That's the death knoll for a young company.