The notion that the WBS is BDUF (Big Design Up Front) is nonsense. The WBS is an emerging document. A simple read of MIL-STD-881A states it is a living a fluid document of the “Product Breakdown” and the processes needed to produce that product.
Rolling Waves are releases, or iteration depending on your choice of words. Rolling Waves are also larger scaled iterations in billion dollar programs or can be scaled to week or month sized iteration.
Requuirements signoff in Scrum is formal during the planning session, just like it is formal at the beginning of a rolling wave for a billion dollar manned space flight program.
This type of post is not only confusing, it has little actual value to the management of a project or program.