After years practice, I believe that there should be a line for
- Whether a change is a new story or changes to existing story
- Whether changes are the change which bring lots of customer advantage or it is another hypothesis from product owner/manager
And if you frequently experienced any problem related to story change becoming new story, you need to looking at what is the root cause of problem. It is very important to solve root cause of problem. Mostly, I found that lack of either functional requirement or architecture design are mostly root cause of problem. It is highly related misconception about agile doesn't require document. Agile means "communication" over "document". It does NOT mean that you don't need documentation. But from time to time, document, esp design document, are very helpful, but skipped. Although you don't need them in a very formal format. But some sketch on whiteboard and record them in somewhere will help team understand problem and find question earlier.