Don't think A's can be upgraded to B or C. Those air intakes are completely different and I'd imagine those are quite impossible to change. B's should be able to upgrade to C though.
And if @PiSigma's uncle's statements are anything to go by, air intake design relate to the structural strength of the airframe so it's not just a matter of slapping on DSI, it's practically rebuilding an entire aircraft and if CAC is going to spend that much effort, they might as well get an entire new J-10C.