New Zealand and Australia have kept their ensigns, although there have been moves to change. Canada brought in the new flag in 1965.
How could Canada keep a Red Ensign as its flag? It doesn't have to be the exact Red Ensign from 1965, just a Red Ensign.
Maybe Mackenzie King gets his design through in 1946?
Or Pearson holds a referendum and it fails? (He chose to send it to committee. Diefenbaker wanted a referendum, presumably because he felt the populace would choose the status quo).