Anyone posing a question is absolutely free to change what answer is the accepted answer at any time. You can return 2 years from now and change the answer marked correct. There is an initial delay on marking something correct. I don't remember what it is. @Ryan state's its 15 minutes in the comment above. That sounds about right.
If you are not posting answers because you see one is already marked, then you should start. If you have, what you believe is a better answer, by all means share it. Even if the one person who asked the question doesn't mark it correct, you can still help many, many users.
If your concern is just the reputation gained from marked answers, well, I sort of think that's the wrong reason to post any answer. But you can often gain MORE rep with a great answer even if it's never marked as the "selected" answer.