From https://meta.stackexchange.com/questions/7237/how-does-reputation-work

> You lose reputation when:
>
 - one of your accepted answers loses accepted status: -15
 - you unaccept an answer written by someone else to one of your own questions: -2


So it looks like the rep they got on your initial choice gets taken back and you lose your little bonus for "completion".

There seems to be lots of discussion on meta for other SE sites about the advantage of leaving questions open because readers tend to stay away from green. It is argued that by removing the tick, visitor flow will increase and if the answer gets two more up-votes, the poster is better off (2 X 10 - 15) than sitting in a question that looks complete.