Is it possible to change the accepted answer? Why should I do it?
Yes, you can. Simply click on the tick of your newly preferred answer.
Sometimes old questions receive a new answer that covers the topic better and/or that, according to the OP, deserves to be the accepted one. If you find yourself in this situation then probably you should change the answer.
Remember that our questions are useful also and especially for new and future visitors/users so if you choose a better answer, it will help others as well. You have a responsibility in this sense.