The SX is NOT compatible with the AR-23 but it is compatible with the AR-C. We cannot upgrade these units to AR-C either.
When the AR-23 changed to the AR-C
1. the firmware changed
2. the board layout was changed
3. the microprocessor was changed
We say that the AR-C will be transparent to the AR-23 users. This is true. It does not mean that you can use an AR-23 where you would use an AR-C, like in the case of the SX. The SX will work with the AR-C but not with the AR-23.