When I and my parents tried to do check-in, one of the staff told me that "there is overbooking so we are not able to stay at the room we booked at the first night". Because of that, the staff offered vouchers so that we could have meal at their restaurant. And then she said that we could stay at the room we have booked at the second night. Since my parents are old I didn't want them to move their luggage so I asked about this and she said if we pack the luggage and leave them in our room they will move them to the new rooms.
After staying one night, we walked around the town and got back to the hotel, expecting the new rooms were ready. However, one of the staff (different staff from the day before) told us there was no room to move so we had to stay at the same room. She knew that I had received the vouchers but didn't know that there would be room change. After I told her about the story, her face looked like I was trying to get something more from the hotel. Anyway she offered another voucher but I refused it since we needed better bedrooms, not free meals (the beds in standard single/double room are not good-sagging mattress).
If anyone encounters a similar situation as mine, you have to ask written note or document from the staff that guarantees your room.