I'm not fully following the question, but the way it works is that the obligation of the institution, in the case of pre-existing accounts, is to look at the accounts and the information the institution has on file. If there is nothing to suggest that the account holder is a U.S. person, then there is nothing for the financial institution, at that point in time, to report to CRA, because there's no indicator to suggest that the person could be anything other than a Canadian.
On May 14th, 2014. See this statement in context.