Changing a ressie within 30 days of arrival is a really high-risk, low-reward gamble for the reasons Doc gave you. There would have to be some very compelling reason for me to ever attempt that one.
Since you're not a member, I'm wondering if you are renting and now want to change. That would be up to the owner you rented from, but I think 99% of the knowledgeable owners would not even attempt to make a change inside 31 days. It's WAY too risky.