I think, not sure, that the model number change was in either 2003 or 2004. The one major difference is the head lights are sealed beam on the B-7510 which really makes the lights much better.
I have heard of people changing the wiring size on the B-7500's to improve the lighting.
I think the major items of the two tractors are about the same, hp, pto hp, transmissions, etc.
They are pretty much the same machine. The loader valve has been changed some and the head lights as WayneB mentioned above. The same loaders were available for both as well. I know of no other changes tho there might be some.