Its not going to be a new desine its just strechted to host a big engine.
second it going to host bigger radar.
third its going to carry more load.
forth itsnot going to be more expensive to the limit they can't afford it it will be at latest 25 m$.
fivth the Swedish actually doing the same with the new NG jas gripen.
Or they just strengthening the body using the ws-13 if it is really more powerful than the rd-93.
and better radar.
Bhai are you serious :/ I mean I could not comprehend it or you just said that just stretching the design will be good enough to accommodate the mighty AL-31 instead of mini me RD-93 and their will be no aerodynamic, hydrodynamics, material strength bla bla bla required to address the integration of bigger engine.
Even if it's possible the way you have said what about the fuel consumption just stretching the internal fuel storage be an easy task ???
What about the lifecycle and flying per hour cost how PAF going to quench the thirst of the mighty Salyut AL-31 and fits the life cycle etc bill ???
Hosting a bigger radar is not every thing :/ ...
Bhai you said it's not gonna be expensive :|, brah! Only research and development for this extension gonna costs millions and price gonna shoot up to equalise J-10 or exceed.
Grippen NG is altogether a different story don't compare it with JFT in the very case you are mentioning. If you wanna compare it this way then it could be like Grippen be switched to GE-F110 an absolutely another class of engine.
Right now Grippen is flying with same engine with uprated thrust that means the same series with uprated version. F404 to F414.
The last thing is yes WS-13 is the right path for JFT similar to JAS-39 and if included with composite body like reinforced carbon fibre, honey comb structures and other special materials it may increase in price moderately but value as well.