The 996 and 997 wheels don't have the same diameter. A 18-inch wheel requires higher profile tyres on the 997 than on the 996.
997 Carrera:
Front 235/40 ZR 18
Rear 265/40 ZR 18
996 Carrera 4S:
Front 225/40 ZR 18
Rear 265/35 ZR 18
(I doubt 996 17-inch wheels fit, as the brakes are larger).
You'd be set if you could find a set of 225/42 ZR 18 tires.