To be clear @JustAnotherWhinger those are two separate things.
Yes, the rules of succession were changed so that it was by birth order not by sex.
But the rule that only William's children would have the HRH and prince/princess titles because they are the great-grandchildren of the monarch AND the grandchild and child of the direct heir, on my understanding, predates this.
Then they all move up with PC became KING and they are all grandchildren rather than just greats of the monarch.
I can't remember where I read this, but I did at the time somewhere.
The thing is that QEII was on the throne for so long relative to other monarchs this sort of situation never arose before, I believe. The theoretical then hit reality and was perceived (by some) to be unfair.