I'll reiterate what was said - it is NOT a roundtrip for purposes of the online booking bonus, but might be for fare construction, depending on the fare.

Example: I had a trip that was supposed to go IAD-JFK-IAD. The return was cancelled due to a mechanical problem, and I got rebooked LGA-IAD. My online booking bonus didn't post. When I called, I was told "you can't get the online booking bonus, because that's only applicable to a round trip, which means into and out of the exact same airports." Fortunately, logic prevailed; when I pointed out that I had been rerouted only due to the flight being cancelled, MP posted the bonus for me.