J-10 is best example. The J-10A design is optimized for AL-31 engine especially the air intake. When J-10B first being introduced, it could fit both AL-31 and WS-10, everyone was asking why haven't they started mass production of J-10B. Then came J-10C with final addition that will go on full mass production.
The same can be said for J-20 as further changes will be made especially when WS-15 is perfected with potential for TVC being explored. To fill the gap, they'll build maybe in smaller hundred J-20 with WS10 tuned up saving budget for revised J-20 with more capable WS-15 that is likely to be far different from present J-20.