Flight codes are meant to signify the particular starting point, stops, and destinations that will be made along the way.
You can change the aircraft and the flight code stays the same. Many flight codes happen on a regular schedule, like daily. But the same flight path and pattern at a different time on the same day will get a different flight code.
Some patterns have many flights per day along the same path — maybe ten or more. But they each get a different flight code.
All of this is within a single airline or codeshare, of course. Two different airlines can use the same four digit set of numbers for totally different flights, because the full flight code includes both the airline designator and the four digit number.
You can change the aircraft and the flight code stays the same. Many flight codes happen on a regular schedule, like daily. But the same flight path and pattern at a different time on the same day will get a different flight code.
Some patterns have many flights per day along the same path — maybe ten or more. But they each get a different flight code.
All of this is within a single airline or codeshare, of course. Two different airlines can use the same four digit set of numbers for totally different flights, because the full flight code includes both the airline designator and the four digit number.