Swapping '13 ecu for a '12 will most likely cause a huge headache that you don't want to deal with and there will NOT be a positive difference since the '13+ models have s+ buttons, better Li - ion batteries, and gearing ratios are different I believe for MT.
Now my question is why would you possibly want to do this?
It is hard to say whether an ECU swap is feasible or not... If Helm would come out with a service manual for the 2013+ models already, then we could at least see if the pinouts are the same. I don't think they can be THAT different from each other, since all of the same basic stuff is there. The differences in the ECUs might just be about small details, like the final drives being different and such...
Ecu plugs are different. 11-12 have 40 pins, 13+ have 49 pins. If you wanted to repun your car body harness and your engine harness it would technically plug in. It may nott like the lithium mcm in the back. Peter has done a nimh mcm in a lithium car and it needed a device to keep the car happy. Id assume the same would be for an ecu swap.