We should almost never change universal values for existing fields. Drupal 7 does heroic work at allowing almost all field settings to be specific to the content type or other entity the field is attached to in a given instance. (Incidentally, that is what Drupal calls internally a unique field-and-entity combination: an instance.) Sometimes, however, we'll want to change one of the global configuration options at the bottom. We shouldn't, most of the time. We should create a new field, in this case, rather than re-using an existing field.