I was under the impression we were talking about new production J-20A/S airframes for whenever WS-15 is not production rate constrained.
Vanilla J-20 airframes that are already produced -- absolutely I expect them to be re-engined with whatever the latest and appropriate WS-10 variant will be.
For J-20S, I suppose I'm of two minds on. I could see them giving it WS-15 because it seems to be a two seat version of J-20A (or rather, J-20A is a single seat version of J-20S), but it's possible J-20S isn't expected to operate in as many of the same regimes or mission profiles as J-20A.