Did they do that with the original? Regardless, this is the only actual expansion, the other DLC is just minor missions and slight changes. This changes the entire campaign, much like Enemy Within did (rumor is even more so.)
Also I wouldn't play the game with the expansion the first time. It will totally change the experience.
I droped Xcom one in last mission when they announced expansion. I was so pissed that I have to replay it again :/