Imagine you ask a question, get some good answers and after some days you choose the best as the accepted answer.
What if days or even months an even better or more comprehensive answer comes up - is it impolite or to select that answer?
For the author of the originally accepted answer that would be a loss of rep. She/he might even complain and downvote your question or at least isn't motivated to answer your future questions.
What would be the fair solution?
Not impolite at all. If there is a better answer for the OP, he should accept it instead. Downvoting or complaining would be a childish reaction.
Anyway, most reputation is likely to come from upvotes, not from being accepted.
Accepting an answer on SE sites means that it is the one most helpful to the asker. This also makes the answer float to the top, so that future readers see it first, regardless of total number of upvotes, unless you accepted your own answer in which case it would sink to the bottom. This helps to make SE a great resource for all future generations.
See it not as a loss of rep, but rather not gaining of additional rep, since this is not a downvote. Personally I changed my accepted answer several times on SE sites.
