We will have to treat such an upgrade in different manners. One cannot call it an upgrade for everything that Block-III. Indeed, we will be looking at upgraded Block-III from Block-II in quality area and especially in the areas of Radar & weapons to the most of extent. Since, airframes are different, therefore, the then older Block-II will not be a 100% Block-III but one can count on the same level in the ring once all hell broke lose. Remember, Thunder design was frozen year(s) ago but it had the quality of adaptation from day one hence, we saw an upgrade of Block-I to Block-II. I hope you got my point that I wish I could express more openly but not at the moment.
The Airframe has the flexibility, therefore, you may have noticed that how Thunder's development & evolution has been coined in the same terms of F-16 fighting falcon baseline & then even we expect a viper upgrade despite the airframe being the older one. Conclusively & for a layman like me, as I am still learning certain things, most of aspects & criteria are never told nor shared to the world at all.
Similarly, we can expect an air-cooled LKF-601E (AESA) for older blocks given the limitation due to size of nosecone but on the same time it doesn't limits the aircraft to perform lesser than the newly built Block-III with KLJ7A AESA Radar. On one point you have a shortcoming in specific bird but on other hand, you have an alternate solution to compensate the shortcoming and in this scenario, putting it simply, remember the netcentric capability. For this point alone, at-least I am very sure that Pakistan is far ahead than the adversary and despite being a rich spending, IAF needs years to catch-up with PAF that already excelling more.
In couple of years or plus 1/2, we will see a totally different structure of Pakistan Armed Forces and especially in Electronics related area. I can imagine a flying castle. In'Sha'ALLAH.
IF AN UPGRADE IS DONE, I don't see it ending up in short like that. Though, it may have a different gadget beneath the skin with differently told specs etc but still, in the end with a different game plan, tactics & deployment; the end result will be a totally upgraded Block-III fleet. That mark will always remain there as the airframe will be quoted as an older block but capability wise, nothing different or short of punch.