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.

Drupal tells us: These settings apply to the Image field everywhere it is used. Because the field already has data, some settings can no longer be changed. This can give us a false sense of security; instead of can no longer be changed we should replace these words in our mind with should really not be changed without being really sure about how what your doing will affect existing data. Really. Because often we still can do things we ought not to do.

Changing instance-level settings, the top grouping of configuration that Drupal names after the content type, is much more likely to be safe.