Firstly, who tells you that WS10A has a same external dimensions with AL31/F?
Then, sir, you need to redesign the whole plane when something as important as an engine to be replaced. It is more difficult if compared with a car, in fact. You need at least to learn something from the LCA thread to know how much trouble would resulted with a replaced engine at first, please. As both J10A and J11A would be finished mass production within a couple of years, the Chinese authority would be too stupid to consider redesigning them just for adopting a new type of engine.
Repeat once more,
- China has hundreds (each of them) Su-27, Su-30, J-11A, and J-10A flying. Even for their maintenance, the country needs to import Al31 at hundreds level.
- Mass production of both J10A and J11a would be finished within 1 or 2 years, thus no need to redesign them for a new engine
- J10b, J11b, J15,......, the new types of fighters will use domestic made engines. All of them.
- China will no longer import new types of aeroplane engines from Russia, in mass, any more. A time is over.
BTW, remember the story of Su-35 import? I declared similarly when somebody aroused the topic here. China will never import fighters in mass from Russia anymore. Lots of people jumped out to challenge me but now the truth has become more clear.
Please wait and see.