The part WAS redesigned completely for '02 but it's not a redesigned part for your '99 car although it might be improved from the original. It is not possible to use the '02 redesigned part on a '01 and earlier car.
Any time any throttle unit is replaced, software must be DLd through Vadis or it simply won't run. Any experienced Volvo dealer anywhere knows this and will get the latest part from Volvo so it's not necessary for you to tell them anything. It's my opinion that whoever you talked to at customer service possibly doesn't have their facts straight if they told you that a newer design '02 throttle unit can work on your car as it's impossible. There's more to the '02 cars system including intake manifold, ECU, etc that makes the idea of that impossible, IF that's what they're suggesting?

I have yet to see a replacement throttle unit fail on the '99-'01 cars so perhaps that part is better than the original? That's possibly what customer service meant to say as the design of that part is the same, looks the same physically. The '02 throttle unit is totally different, new design, different manufacturer, different intake with new location and is obviously not even similar in appearance. The only way to get a car with that part is to buy an '02 car.