I think the first and most fundamental change Bill C-59 makes is to retroactively eliminate the right of requesters to access information under the act. In fact, it has the effect of retroactively eliminating the office's entire investigation, all of the evidence gathered, and requesters' right to appeal to the Federal Court. It also retroactively eliminates all potential liability, be it administrative, civil or criminal.
The biggest change probably isn't how it will affect the mandate of the Office of the Information Commissioner but, rather, how it will affect Canadians' right to access information and their right to hold their government accountable for its actions. That's the main consequence of Bill C-59.