I sent an email to ask them three days ago, but they haven't answered yet!
I don't think they can give me an extension considering that my offer is for Spring 2015.
I haven't found anything about the withdrawal in the admission letter. I found this in my letter: "New students may register beginning Friday, February 13, 2015". Also, they asked for a deposit by February 28, 2015. Doesn't asking for a deposit mean they expect some students to withdraw after accepting their offers? I'm wondering!
Thank you Feanor for your reply.