I don't think 'switching' is the correct term. No contracts are required for WBFF, or NABBA, or WABBA, or NAC, or Musclemania, which all have a pro class at their world championships, or even the pro only Battle of Giants contests. If a bodybuilder has not been under recent contract with the IFBB, such as Lee, or Milos, or Rodney, or Tommi then there is nothing being switched.