Yes, while it relatively simple to increase thrust by raising temps in the hot section, those parts need to be upgraded to handle that increased temperature and thrust...It is not as simple as sticking a new engine on a flanker. With the higher thrust, the entire plane must be re-designed to strengthen the body. It must go through extensive testing.
The question is, is this the best use of resources? I think it is better to just produce more J-20s with all this resource instead of upgrading a flanker with a new engine.