Requirements: elaborate on requirements change

[amyjko]

When defining requirements at the start of a project, how much should you actually stick to it? For example, if you plan requirements at the start but then new things come up, then logically the requirements should change. But if that's the case, why not just build requirements incrementally from the start? I get that having requirements helps you plan when building a project but it seems like it would lead the development to be more inflexible.