Do you get moved down a section on a USCF chess tournament if your score is low? For example, I would like to play a class championship tournament. I'm a class B, but I registered for the Expert class section. If I don't score enough points to play anyone in my section for the rest of the tournament, will I get moved down the class A section? 

Could someone explain what would happen? 

Master
U2200 
U2000
U1800
U1600
U1400

etc.
You do not get moved out of your bracket.  If you're a B-player in the Expert section, expect to finish at the bottom.  I'm not even sure if you're allowed to 'play up' unless you play in the open section.