For example, if we have a flight with a code SU0102, does it always goes from Sheremetyevo to John F. Kennedy or the origin and destination points can change for this flight? The same about schedule: does a flight SU0102 always leave at 15:40 and arrives at 17:10 or the hours can change?
In short words, can the schedule, origin and destination change for a flight with a certain number and if so is it frequent?
There is nothing anywhere to say that the airline won't change the flight number or reuse the same flight number to connect different destinations.
Case in point: United flight 237 contains 2 different segments flown by 2 different aircraft today so in essence you can look at it as 2 different flights.
On occasion airlines change flight numbers for the same destinations due to various conditions most notable one is an accident for example Moscow - Hong Kong is now flight SU212 but it didn't used to be and so on.
Just about the only thing that you can expect is that flight number for a regularly scheduled flight will change sufficiently enough in advance of bookings/reservations so that airline doesn't have to notify clients of the change midstream.