May I ask the reason for swapping to -600’s? If you are already operating a -500, then your pilots and engineers will all be rated for the -500’s, and any spares you have will also be for the -500’s. From a commercial perspective, your customer will see no difference in the product they are flying on, as there is almost no change in the back of the aircraft, so you would be paying more for a product that is no different for your customers. The biggest difference between the -500 and the -600 is in the cockpit, so you have to ask whether it is worth operating a more expensive aircraft (in lease rates) just to have a more modern cockpit?