Disclaimer: Couldn't get anything on the "search"

What do flight numbers mean and why do they change? Today I received a "schedule change" email from NW for an itinerary I have in May on what has been flight 1886 DTW-MHT. The schedule change is simply that the flight departs now 3 minutes later, at 8:58pm (versus 8:55pm) and arrives 2 minutes later, AND the flight # is now 1585 instead of 1886. Same equipment, just a new flight number and 3 minute schedule change. Why?
What do flight numbers mean and why would it change for a 3 minute time change? I take the flight often and it's called 1886 most of the year, but periodically changes for awhile. Just curious.