A Google AdWords advertiser says that he would rather prepay for his Google AdWords rather than have it set up as it is now, with him paying for the ads after the fact. He asks in a Google Groups thread whether it's possible to change the account from post-pay to pre-pay.
AdWordsPro says that it is not possible to do so without closing your account first.
AdWords accounts can't be changed from post-pay to pre-pay billing. If you'd like to switch to the pre-pay option, you will need to close your current AdWords account. Then, you can set-up a new account with the pre-pay billing option that you prefer.
The link below from the Help Center has more details about changing payment options. http://adwords.google.com/support/bin/answer.py?answer=34103
I have to say that this option is a bit inconvenient for people who maintain huge AdWords campaigns. Would it be possible for the Google AdWords team to make it easy to do without having to close an account and recreate it from scratch?
Forum discussion at Google Groups.