It’s impossible to quantify the ease of room-switching. You and your roommate should call ResLife/ResEd ASAP to find out if you can switch before you move in. Otherwise, you have to wait 2 full weeks after classes start.
Regardless, you might not be allowed to switch halls “just because.” The staff goes through a lot of effort to place all of the students and allowing switches on a whim would be very time-consuming. Best of luck – call and ask as soon as you can.