Change of orbit with change of mass?
What you said :


  monoliths increase the mass of Jupiter


What you quoted :


  
    these monoliths increase Jupiter's density
  


Increasing density is not the same as increasing mass.

By squashing stuff together you can increase it's density without affecting it's mass at all.


  But if Jupiter were by scifi magic made as massive as a star, would its orbit remain the same regardless?


No, nor would any body in the solar system, including the Sun, remain undisturbed.  It would be a massively complex change.