Rewrites are great. You have a specification that is so defined it is literally code.
When it's blue sky, it's harder. Plans will be wrong. The users don't understand really what they need or want. It all ends up evolving. Anything with a GUI is worse because users/customers need (want) things moved about, re-themed, with no regard to what's below. Best to nail them to mock up designs they signed off on. Same with API interfaces. If they signed off on the design, you can then point out "spec change" and get more time/money. It's more about ass covering than using the outcome or process.