Although I’m not “gg”… I think you made a bad choice. It’s important to follow convention/rule even if it seems unfair sometimes. In any case, I don’t think that you need to worry too much about your credit or your record. In fact, one month rent fee can’t be that much money to damage your credit anyway. But, I would like to suggest you to reconsider — you’re taking a very bad move. Just pay what you have to pay and move out soon.

이번 렌트 안내고 나가는 건 그다지 크게 문제가 되진 않지만….
얄미워서 몇달 더 개기겠다는 건… 정말 무대뽀군요….
제가 보긴 원글님쪽이 잘한거 하나도 없는 데요.

First of all, I am not a lawyer. I am just a renter-sympathizer because I was once a renter with my husband and children. You cannot depend on my word as legally binding advice. If you check the phone book, there would be a lot of real estate lawyers.
Written notices are always better evidence than phone messages. If you want to move out on 11/15, which I think is reasonable, I would call the landlord and tell him so but I won’t leave message on his machine. If you want to stay longer, I would ask the landlord politely if he can give you the contact number of the new owner, so that you can negotiate with the new owner for longer stay. he might not want to give you the info in the fear that you play menace in the real estate deal or because the new owner wants to evacuate anyway for the repair or for his own use. If there is other tenant renting from the same landlord, check how they are doing.
Either way I won’t leave message on his machine.