If user A comes along and answers your question, and you accept it, he gets 15 reputation. If user B then adds an answer and you accept his answer instead, user A will lose 15 reputation and user B will gain 15 reputation.
This can cause hard feelings (it is amazing how much people care about stupid internet points!) so what I recommend is waiting awhile before accepting an answer. Often times a better answer will come along.
Don't over think this, the accept mark is yours to do with what you want (okay, so there are rare exceptions, but those are rare enough I won't go into them). If you accept user A's answer, it is perfectly fine to leave it that way, even if user B adds a better answer later. And, on the other hand, it is perfectly acceptable to unaccept user A's answer and accept user B's answer. The check mark’s only purpose is to show what helped you and solved your problem, it doesn't matter what order the answers came in.