I have no idea what their policy is but I can tell you an incident that happened to me. I was flying to Baton Rouge, there was bad weather, we got diverted and we landed really late. I got to the counter a bit after midnight (I think it was just past closing time and it was a holiday). They had already cancelled my reservation, and told me I couldn't have my original rate because it was after midnight. The agent at the counter told me that I'd have to do a new reservation as a walk-up at the counter. It was too late and I was too tired to argue, so I did it and paid a higher rate. I was quite annoyed about it though and I should have complained.
I suspect the agent was trying to finish up for the night and didn't want to be bothered.