I don't know what the collective bargaining agreement says, but I suspect that Shaq couldn't "take less" at any point during his current contract. An extension might call for less money in the future, and perhaps it might be allowed to re-structure the contract, provided that the end result keeps at least the same amount of money on the books, but I have my doubts simply because I can't see the players union going for that.
And if this were tried, I'm almost certain the players union would be up at arms.
Lurker would know better than I. He's the one I consider the CBA expert.