Currently as it stands, once you open a bounty for a question, and once you've accepted an answer for that, you can never ever unaccept it anymore. And if a better answer appears or if you realize that you've make a mistake in accepting a wrong answer, you are out of luck (see this question).
To me this is as unreasonable as auto-accept answer after bounty period expires. But I don't know what others think. I'm putting this up as a feature request.