This is actually possible right now.
Each reply on an Answer type thread will have either a 'Mark as Answer' link or a 'Marked as Answer' label in the top right corner. If you are the creator of the thread you may either select a different reply as the answer by clicking the 'Mark as Answer' link or you may click the 'Marked as Answer' label on the reply that was chosen in order to remove that status from it and change the Thread's status back to Unanswered.