For the benefit of anyone who finds this topic - the difference between the two models is that the newer one has a re-designed midplane PCB which allows greater airflow to the devices in the IOM/PCM slot to handle the slightly increased cooling required for the embedded controllers vs SAS expanders.
There is no way to force it to run in the older enclosures - with access to the developer resources, I have tried. ONTAP checks the part number at boot and refuses to boot in the older one.
The older ones have "Copyright 2008" visible on the midplane when the left-most column of drives is removed, and they were used for both DS4243 and DS4246 shelves.
Yes, all very complex, but this is why we don't support converting a shelf to a controller, only the other way around.