Originally Posted by nerd
What's confusing?
The rationale is that they re-number one of the flights so that 2 flights won't be in the air with the same number at the same time.
Why not re-number the second leg, if/when the situation warrants it (e.g. if the first flight is delayed)?
One possibility is that they do this for internal purposes as well, perhaps having something to do with crew scheduling issues. In the example the OP discussed, there's a change of aircraft type at IAD, so maybe they use the modified flight number for crew bid purposes as well as the two flights in the air at the same time scenario, but this is pure speculation on my part.