So we estimate the story and put tasks on the board. Then during the Sprint the UX guy comes up with a user experience for the story that is more complicated than what we originally thought. So on one hand we want to implement the right feature for the user, on the other hand if we had known about the complexity of the UX during Sprint planning, we probably wouldn't have committed to the story.

So what to do in cases such as this?

The same question applies to cases where a usability test or user feedback during the Sprint shows us that we should change the feature.

In general, where do you draw the line between clarification of requirements or adaptations and the changing of scope?