on a ticket there is always a place where the ticket price is broken up which is usually the point of turnaround. So if you ticket was say 1000.00 dollars it would show yul-fco 500.00 and the return would show fco-bru-yul as 500.00 dollars. So what I am guessing they did is they upgraded your ticket to a fare that allows for a stopover. Because you have started the return portion of your jouney fco-bru they can't reasses the fare as it is calculated as fco-yul.
Without seeing the ticket this is just a theory.

Now why they won't let you upgrade I have no idea.