Not always, but often. For example, the YYZ-ORD numbers were consistent, but the continuing segments, like today, often changed. So for example, flight 1111 was always the ~5:00pm departure from YYZ to ORD. I can't remember the continuing destinations exactly, but they did change, so one day the continuation of 1111 might have gone to MSP, while another, it may have gone onto SAN. This changed sometime around 2007 or 2008, so well before the merger, and then the numbers kept changing.