12.3 looked to be aimed at MCU1 cars (No-AP,AP1, AP2.0 and very small set of AP2.5)
12.4 looked to be aimed at MCU2 cars (AP2.5 and AP3.)
12.8 seems to be MCU1 cars again.
I don't think the upgrade path is 12.3 -> 12.4 -12.8. I think its split
MCU1 12.2 -> 12.3 -> 12.8
MCU2 12.2 ->12.4 -> ???
I thought someone had mentioned in another post that 12.4 was to patch a sensor beeping problem after HW3 retrofit