Do people ever manage to move from Boutique's to MBB? If so, would some time at a Boutique be a good stepping stone from a strategy role at PWC?

I have the same question

I asked the same question on another forum and the answer I got (seemed from a boutique insider) is that it rarely happens, and the reason that he gave me is that MBB don't want to re-train people. But I have been holding my doubt about his reason. So I really want to find out what people here think. Thanks a lot!