PL-15 will still be used by PLAAF/PLAN-Aviation until (1) they've depleted all inventory and (2) they've upgraded all their fighter jets to adopt PL-16.
PL-15 may still be produced in small scale to meet requirement of export orders, especially to Pakistan AF (J10CE and JF-17), and other operators of JF-17. However, if all foreign users of PL-15 have upgraded their fighter jets to use PL-16, that will end PL-15 production.
Third scenario is PLAAF stop using PL-15 and China stop producing PL-15 right away. All existing inventory of PL-15 be kept in reserve to meet requirement of foreign users. Admittedly, this is very unlikely to happen.
The "make use of full capability of PL-16" argument also apply for PL-15. We know for a fact some older plane could not make full use of PL-15, but can still use it. At minimum PL-15 is better than PL-12 even without the full capability. Same argument apply to PL-16 vs PL-15.
At this stage I don't see a reason to believe that aircraft that can accommodate PL-15 would need an upgrade to accommodate PL-16.
For the ranges that these missiles are operating in, they should be network centric by design at this stage and all the aircraft that we know can currently operate PL-15s should already be running datalinks that are rather high end and should comfortably accommodate PL-16, I imagine, and at most may just need some lines of software loaded.
But I would be surprised if a hardware avionics or radar upgrade is needed.