Yes, you're right. Until there is a constitutional amendment under section 41 of the Constitution Act, 1982, the powers and the office of the Governor General really cannot be affected. So she has the ability to exercise her reserve powers either to agree or disagree.
But what I think has happened with the Governor General over many decades, if not centuries, is that he or she is acutely aware of the rules of conventions. That is why, in certain circumstances, the Governor General has refused to accept the advice, such as in the King-Byng affair.
So it is very important to stress that conventional rules are not legal rules; however, they can be as binding on actors as legal rules.