License agreements were never a "sacred cow" for either the Chinese or the Soviets.Naturally, how could you not expect the parent company to have introduced improvement of the type before the licence manufacturer?
They _tried_ to set up production, but failed. The plant in Shenyang assembled five MiG-19P, transferred from the USSR in disassembled state, then a year assembled one MiG-19P of its own production, the quality of which turned out to be unacceptably low. After that, the plant in Shenyang began to produce a copy of the MiG-19S. In Nanchang five MiG-19PM, received from the USSR in disassembled state, were assembled, then 7 aircraft (of unacceptable quality) were produced for two years, and further work on the J-6A continued in 1974.Actually, the Chinese did build the MiG-19P under licence.
These were MiG-19PMs, most of which were built after 1974 - according to some reports the Chinese were helped by the Albanians who transferred several of their MiG-19PMs to them.For various reasons the Chinese took longer to develop an indigenous industry; the Great Leap Forward was a disastrous leap backwards for China's military industry development and the first J-6s, or Dong Feng 102s were MiG-19Ps, but they were so badly put together that the PLAAF rejected the lot of them and only 33 were built.
Nevertheless, in 1957 the MiG bureau created on the basis of the MiG-19 the SM-12 front-line fighter, which was significantly superior to the MiG-19S in most characteristics. It was supposed to replace the latter, but due to the whims of the Air Force, testing was too long. In 1959, two SM-12s were armed with K-13 missiles (Sidewinder copy)Nevertheless, unlike in the MiG buro and the MiG-21 overshadowing MiG-19 development, the J-6 programme continued despite the growing investment in the more capable J-7 programme.