CASA guarantee to make any upgrading of an AOC, addition of a complex type to an AOC, and just about everything to do with an AOC, into a major exercise.
Moving from B1900 to Dash 8 involves not just simulator programs, check and training approvals, engineering approvals, ground handling assessments, proving flights etc etc but a whole new monster called flight attendant training. This attracts the attention of CASA staff dedicated to the subject like you can not imagine if you have not seen it in action. Also CASA finally appear to be taking a dim view of operators who plagiarise other operators' manuals, so dreaming up new ones to comply with what they want this week can be a hell of a task.
Buying an AOC if it is cheap enough and can be done legally, would be a whole lot less painful. Having two AOCs is not entirely silly either. In the event of an accident or some other mishap, one can be sacrificed without losing the entire business.