I agree that it seems like a drop in the bucket to rebuild (or at least significantly remodel) the existing restrooms so that they fit the new surroundings, but we do have precedent that would imply they may just keep them as-is
When DCA transformed this...
...into this...
...the bathroom interior still looked like this:
It's not like DCA's redo was done on the cheap (rumored to have cost ~$1.2B), yet these extremely out of place bathrooms remained. Although the surrounding area had been rethemed to a beer garden-like area, and Route 66 had been relocated to the other side of the park, the interiors were unchanged. Although it's great to have high hopes for Avatar, I wouldn't expect the bathrooms to get many changes