Agree.
For one, their titles will definitely change. The first half of 1999 announcement is that Edward will get his father's dukedom after it merged with the crown. Hence the second part, naturally as an Earl his chidren will have the courtesy titles as the children of an Earl.
But then what will happen after he's a Duke? Surely Louise and James will also have the right to have the courtesy titles as the children of a Duke, right? Whether they use or not is different matter (case point, David still use Linley instead of Snowdon).
Now about HRH Prince(ss). I always wonder whether the second part about the children's title is there partly because Edward is an Earl and post George V, none of monarch's grandchildren whose children of an Earl were prince or princess but of course it's also because they're from female lines (Mary and Margaret's children).
Even after reading iluvebertie's reply in BRF Titles thread about LP, Warrant, and Will, honestly I still find it strange that if it supposes to be permanent (removing Louise's and James's right of prince/ss title), The Queen only put it as a Will. She and George VI issued LP to grant prince/ss titles to (the then princess) Elizabeth's and William's children which essentially making exempt of George V's LP, both only apply for one specific person. Okay, in both case, it's granting titles and not removing it. But then we have George V's LP which essentially removes the rights of royal titles of the future monarch's grandchildren from female line and great-grandchildren (other than the first grandchildren of PoW), making a change of Victoria's LP. So it's LP for LP. So maybe Sophie is right because when Louise (and later James) is 18, Edward already get the promised dukedom because most likely Philip and The Queen are no longer around.
But still, King Charles might choose to keep it that way (no prince/ss titles) especially if they're not working royals or might even go further by making it as predecent and issuing LP stating that monarch's younger children would be earl instead of duke and no HRH prince/ss for their children (in the name of slimming down). Still, even if they can, I don't think Louise and James will care much about royal titles and they might opt to not have it (less headache
).