I'm not very optimistic about WS-15 program and reason is we have not seen or heard anything new about it for years .
That is not a reason to expect the engine to be far from completion.
Secondly , you have to differentiate between testing while developing ,and final testing . When you do periodical test of still unfinished product you have to expect changes of parameters . In this case , engineers working on J-20 would have to expect that in some point of time different engine will be installed instead of AL-31
There is no such differentiation because there is no such thing as testing while developing. Perhaps you are thinking of research phase, but that precedes the design stage and in fact precedes the entire project. Main development takes place within the design stage, where solutions are formed to meet those performance parameters. What comes afterward is manufacturing. Testing comes after everything else to verify those parameters can be met.
With a decision made to work with AL-31 as a temporary measure and the WS-15 afterward, the J-20 would have been designed with two sets of parameters. That is not the case of changing from one set of parameters to another.
It will take time , that is certain . Same with WS-15 , when it is finally completed .
Both engines take time, but 117S will take more. The performance of 117S is an unknown until purchased, and one cannot design something around an unknown. The performance of WS-15 is known, because China designs the engine and has access to all the functional parameters before an engine is even built. As a result, to adopt the 117S engine to J-20 now will require a lot more time than WS-15.
Wrong . Good engineer will expect that in some point of time his product will have to be upgraded . In this case (J-20) new engine , probably new avionics , maybe new RAM etc ... That is why you need to strive for modularity and for creativity in design , in order to anticipate future redesigns .
Wrong. What you have said has nothing to do with modularity or creativity. What it really is is "lack of foresight".
A good engineer will take into account of as many potential problems as possible. He will not depend on something in the future that he has no control over to resolve his current problems. That is why a decision is made to have the J-20 first flying with AL-31.
As for dimensions , it would have to fit in J-20 . 
