I have read etiquette for accepting answers on meta.SO but it doesn't address this particular point.
I posted a question and after some days (or maybe longer) I accepted an answer. Later a better answer came along. (1) May and (2) must I change which answer is accepted? The argument in favor of moving it is that it's a better answer; the argument against is the effect on the original answerer's rep -- after all, that answer was good enough for me to accept it once.
(I have up-voted both answers, of course.)