This absolutely. As understanding evolves, requirements evolve. Some domains are well-understood enough that this process can be done up-front a-la waterfall, b

This absolutely. As understanding evolves, requirements evolve. Some domains are well-understood enough that this process can be done up-front a-la waterfall, but more often these domains are so dynamical that the only way to understand them is incrementally, one dynamic (or metadynamic) at a time.