It obviously depend on the language and your tooling too, but I find that it's generally much easier to refactor an under-designed code to add functionality, than trying to refactor a code that was over-designed but doesn't fit the spec anymore.

For example, in Python, there's no need to go for a class when a simple function does the job. And once you do need a class, it's fairly trivial to swap one for the other, especially in a good IDE.