Retractable is kind of misleading. They are looking at a roof that could be "convertible" meaning they could at the cost of a few million dollars a year. Remove solid panels in the spring and re-install them in the fall. Making it an open sky stadium in the summer and enclosed in cold season.
This is a very interesting shift in thinking by the Alouettes and the RIO. For a long time the Alouettes were treated like third rate tenants by the RIO. Now it looks like both parties are looking at each other as partners out of self-preservation. A renovated, comfortable, well appointed Big O IMO would be a good solution for the Als. Moving around the island of Montreal has become a real issue, at the very least you could see a split schedule where the team plays its home games in June, July, August at McGill and then moves its games to the Big O from September to November. This would make McGill very happy as they do not like having a pro team on location during the busy school semester and it would make fans happy not to have to worry about weather when purchasing season tickets.
Also, looks like Montreal will be capable to hose a Grey Cup in a fully renovated Big O by 2024/2026. This is very important to the future of the league and the team.