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Right click B2, select Replace and use the layout option. From the window that pops up, select B as the replacement. You will have to define how B was placed in the assembly, but each child of it. Even though B and B2 are the same to us, Pro/E sees them as two completely different entities. Even though they have all of the same features in the same spots, they are all referenced internally differently which is why you will have to redefine its position in the assembly.
If there is another way, I'd love to know about it.