If a custom script is what he wants, ask him for his opinion on accuracy. Nothing like this ever goes 100% according to plan so make a plan to clean up. The issue here will be was it worth doing custom script versus just moving everything by hand and ensuring it's all correct. Personally I'd be more inclined to move everything by hand and review each product. I'd do it in bite size chunks over months so it wasn't too cumbersome. This way I'd get to touch everything and review it.