Thus far I have not heard of any way to change one's style once it is chosen.  That is not to say that there isn't or that there won't be.  But I nor any person / monk I know has ever found a way to do it.

Truthfully, I don't think it is that big of a choice right now. I am Harmoniuos. I have grouped with Dragon style monks in small groups. He took aggro when we needed him to have it, I took aggro when we needed me to have it. I tanked better then him mostly. I did not see any appreciable difference probably due to my groups not being AE groups. I truly believe your equipment and play style (solo vs small group vs large group) will have a larger impact on your success/progression as a monk in any style. Always play to your strengths.

They added a quest in beta that allowed you to switch styles, but we were told that in release you would not be able to switch your styles and that it was a permanent decision.

you can change your styles.  i have changed once from drunken to harmonious and a fellow monk friend has changed between all 3.  i have no clue as to how many times you can do it but it can be done.  simply make another visit to the master in khal that originally started the quest and he will send you on a path to switch. it requires quite a bit of running around but it is not difficult other then time.