It's understandable that changes sometimes need to be made to improve devices that are already cleared and being marketed.  Are you wondering if it's necessary to submit a new 510(k) for a device in this situation?  There are draft guidance documents issued by the FDA to help companies navigate these questions, and recent updates have been made to the recommendations.  So if you're unsure how changes affect your device status, make sure to revew the guidance documents.  See the recent press release from the FDA about this topic at http://www.fda.gov/NewsEvents/Newsroom/PressAnnouncements/ucm515174.htm