While on a student 500 visa, a dependent applies for a 491 (family sponsored) visa

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1

Hi ,
While on a student 500 visa, a dependent applies for a 491 (family sponsored) visa.

  1. is this legal ?

If yes ,

  1. What happned to primary student holder
  2. can change his visa to 491 if we are successful in our application.

Thanks in advance.