On the surface this seems like a poorly worded ability, but I think that it actually covers itself. It says that any natural weapon you already had deals damage as if it were one size category larger. So it doesn't say that you can take a move action to increase the size of a weapon you already had, it just automatically increases. I think that the first part of the ability can't get NI weapons because it mutually disqualifies you from the first ability by qualifying you for the second though you don't get the benefits of the second because you don't meet it's prerequisites.
So if I polymorph into a bear I take a move action to grow a gore attack, because I now already have a gore attack I can't grow another, but because I didn't already have a gore attack it doesn't grow in size. Also my claws now deal 2d6.
Now I agree that you can still grow a tail, a wing, a sting, a bite, a tentacle, and maybe even a slam attack, but anyone who is going to listen to me probably wouldn't do that anyways.
But who knows, maybe I'm just full of it.