The bounty action is a one way ticket. Once you buy it, you cannot change your mind later. The selected answer becomes irrevocably permanent. If you don't choose an answer and there is one answer with at least two upvotes, then one will be chosen for you and it cannot be changed. If there is no answer with at least two upvotes then no answer will be chosen, and you will not be able to select one later.
However, you are still free to edit the question or add comments, so if the bounty has locked down the answer, but you feel a better answer has come along, feel free to add comments, or even point out the better answer at the bottom of the question with a link.
If you believe that the selected answer is flat out wrong and may lead to errors in the future, flag the question and make a note for the moderators with your reasoning. They can fix individual problems like this when absolutely necessary.
Otherwise, once you start a bounty you are locked in to the consequences.