There have been some other threads on this, one very recently. I don't really have an opinion on the right approach for this, but typically AC will not make changes to connecting flights until the original outbound flight has departed (or possibly not until it lands). AC will often only rebook you once they have a better idea of what flight you can actually take as:
I seem to recall that thread, too; and looking for it. Will look for it, thank you. I think most people would be miffed at being rebooked sooner rather than later, so AC was working on that assumption.
- The original connecting flight can be delayed
- The flight you get protected on can be delayed/cancelled
- The original outbound flight can make up some time in the air
- The original outbound flight can be further delayed or even cancelled, etc.
- Sometimes operations will hold/delay a flight if there are enough delayed connecting passengers (happened on 2 flights I was on last week)
I wasn't able to find the recent thread in my initial search, and don't have time right now to try other searches, but IIRC the person in that thread asked in the lounge to change a connecting flight because their outbound was going to be delayed. In the end, they were able to make their original connection when at first it appeared they wouldn't be able to.
As I said earlier, I don't really have an opinion on this practice. I understand the piece of mind that can come with being protected in advance, but understand why the airline doesn't do this.
Look at this from another viewpoint if things had unfolded slightly differently... Say AC had rebooked you in TPE onto a later flight the next day hours later. You land in YVR and find that your original connecting flight was delayed due to late arrival of aircraft. You try to switch back to this flight but can't because the seats have already been reallocated to people. Now you're waiting in YVR with an unnecessary further delay. You might have been ok with this, but most pax wouldn't be. While this situation is hypothetical, it can certainly happen. Being proactive can sometimes work against you too.