Never ever let them swap seats. There is a reason for this. A classroom goes from a good working environment to a party atmosphere the minute they get to sit just where they want. I've seen it happen too many times, so when they ask, I always say no.
Today I had a group of mostly 9th graders. And they really wanted to sit next to their friends.
"Let us, and you'll be our favorite sub. You'll be the coolest sub ever!"
Oh wow, really? Because I live for the approval of a class of 14-year-olds. I think I actually said this to them, albeit with heavy sarcasm. The sarcasm was heavy enough for them to feel it. They stayed in their assigned seats.