Can I move an exhibitor to a different booth, after they've already been assigned to a booth?

There is not an easy way to do this without refunding the vendor. You can work around it by changing the name of the booth the vendor is assigned to. 

For example, let's say a vendor is currently assigned to booth 105, and you wish to move the vendor to booth 205.  

1.  Visit Convention Tools > Exhibitors / Sponsors

2.  Click on Booths

3.  Rename booth 205 as "205temp"

4.  Rename booth 105 as "205"

5.  Rename booth 205temp as "105"

Generally, this is a process you'll only want to use when moving a vendor to a similarly-sized booth (as there is sometimes a cost difference if moving to a larger or smaller booth).