Well maybe because the old ones or the original design required better engines. For sometime it was mentioned due to under-powered engines of the newer batches had lot of weight cut. The few fully equipped Z-10 in service probably use a 1500 kW turboshafts acquired during prototype stage but now they are stuck with 1000 kW turboshafts.
No - sorry, if my reply was misleading - ... my point is why some think these aircraft with the old-style airframe are regarded the latest production version, featuring a new targeting system ?
IMO it's quite simple ... there were the few prototypes followed by several more pre-serials all featuring this "old-style" airframe, which had to be modified like You mentioned. The result was the revised version Z-10A that is now again modified or succeeded by the latest Z-10K.
In order to keep the pre-serials in service, these were modified to Z-10H standard ... still retaining their original fuselage/airframe. But why are they dispersed around several units, that's a point I do not understand. IMO it would be easier - esp. in mind of maintenance - to consolidate them into one unit.
Deino