At least on the naval side, an introduction of a new platform always came with another batch of the previous generation. The second batch of the 052C while 052D was entering service. More recently, more 075 orders while the 076 is getting readied for service, more 054A orders despite the 054B, 093B despite the 095 being in construction.
PLA hedges for unforeseen teething problems with the new equipment. Sometimes, high-low mixes emerge from the procurement of the previous gen. For example the PL-12 production is still continuing despite the existence of PL-15. In other times, the previous gen is pushed to other roles. The example for this would be J-16. It is not the premier A2A asset anymore. But it will do great as a striker and defensive counter-air asset.