This is not a permanent change, and you are not stuck with only one faction, Instead, you simply put on the tabard and la! You are the new Champion.
We were told that if you run a Coilfang dungeon wearing a Lower City tabard, you will gain the usual reputation for the Cenarion Expedition from the dungeon, as well as Lower City reputation. Supposedly, the tabards, usually acquired at exalted, will be moved to friendly or honored.
While this makes me and my hard-earned tabard collection a bit sore, the prospect of earning reputation for any faction that I need, without running their dungeons over and over, is quite appealing. Here are the Champions we know of so far:
- Argent Champion
- Ebon Champion
- Champion of the Kirin Tor
- Wyrmrest Champion
To me, this seems to call the original quotation into question, as the current description makes it sound as though, instead of gaining reputation from the dungeon, and from the tabard, your dungeon reputation will simply count toward whichever faction's tabard you are wearing.
Of course, that is still mightily handy. We'll be able to run the dungeons that contain the gear we need, while acquiring the reputation we desire.