All the more reason to use an agile approach! The only reason you would be making those major changes is in the face of new information: a change in requirement

All the more reason to use an agile approach! The only reason you would be making those major changes is in the face of new information: a change in requirements, an unforeseen challenge, etc. It's impossible to foresee those all in advance—better to adopt a system that is structured to anticipate them. "Unknown unknowns" are a fact of life, no matter how disciplined we believe we are with our preparation.