The window to change it is wide (I’m not sure exactly how long, but at least into May) Since you are registering early, it doesn’t matter. You’ll likely get whatever you pick. My daughter registered in April (she is in SCA so they didn’t have EA) It opened at 9:00 and the key is to get through the questions as fast as humanly possible because when you are done, you get a timestamp and that is what is used for preference. After you get your timestamp, you can go back and change it without affecting the timestamp.
My daughter wasn’t sure she wanted McCarthy because she was looking for a social environment. On the day that she registered, she had Birnkrant as her first 3 picks and McCarthy as her last 2. She was also accepted to Barnard and we went to admitted students day there after the USC housing registration opened. We took the housing tour at Barnard. They don’t have AC and it was only 70 degrees and my daughter was DYING. So we went home and she switched to her first four choices being McCarthy. 
She got her first choice and it has been wonderful.
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