Each athlete account can be a member of only one gym at a time. If you change gyms, you can switch to another box in SugarWOD in the Settings.
If you need to have access to multiple gyms, you can create a new SugarWOD account with a different email address. These accounts can also have separate permission levels. For example email@example.com can be an athlete at "CrossFit 1" while firstname.lastname@example.org can be a coach at "CrossFit 2".