I am pretty certain that WS-10 even AL-31 would be able to replace with WS-15 relatively straightforward ..... remember that J-20 was designed with WS-15 in mind
Let's hope so. Could certainly be done for MLU of those first variants. I've read the WS-15 does not have larger dimensions compared to WS-10 but refitting Al-31s with WS-10s are difficult enough already that PLAAF doesn't even bother with. Maybe it's an issue of having spare Al-31s around and not really getting any more benefit by replacing them with WS-10s.