Back in April at SEW Blog, I reported Change Your Time Zone Once & Only Once At AdWords because if you make a mistake, Google said you are stuck. Well, of course there is a way out.
AdWordsAdvisor at WebmasterWorld says;
This can be adjusted once by the support team, DamonHD, so contacting AdWords support should have done the trick for you in terms of changing it around. I apologize that they have not responded. It might be worth checking your filtered email to see if a response may have been sent to a spam folder for some reason. And also worth writing in once again.
So it appears that if you made a mistake with changing your time zone, you can still call AdWords and have them update it for you (once?).
Forum discussion at WebmasterWorld.