Hi Missie,
From my contact with the NIB manager I've been dealing with and my reading of the article, I understand that if an offer was made by NIB before the 1st August, then the older rates apply.
However, as Clubman says, the T&Cs say that the bank can then change, so it's quite possible that they aren't honouring this verbal commitment. Certainly worth arguing over anyway.
For interest, is it still worth your while making the switch if the less attractive rates apply, especially when you include legal fees etc?