Thanks for all the responses. My best bet would be to just change to the red-eye flight I wanted, but can't seem to show any availability for it searching on Aeroplan.com. I actually seem to remember from when I was trying to make the original booking that the problem isn't availability on the YYC-YYZ segment, but getting an award seat on the YYZ-BOS segment.

In any event, I just got off phone with Aeroplan themselves to see if things could be changed, and their computers are down for the next hour (this was about 9:15 am).

I did get to ask the agent about changing the routing, e.g. connecting to a red-eye in YEG or YVR so that the itenary actually starts same-day as my confirmed set of flights. After she checked with (presumably) a supervisor, my understanding of what she said was:

Yes, you can change the routing, as long as same departure and arrival points. BUT

At some point in the journey, an agent will have to complete a FIM (flight interuption manifest). This may not necessarily be done at the departure airport but might have to be done by an agent at the intermediate connection spot.

IF for some reason the journey gets interrupted and the intermediate connection gets broken (just can't get on the red-eye out of YVR for example) and I would have to be rebooked for the next day (e.g. same-day +1), then, as I understood it, it becomes a reticketing issue, and there would be a $75 rebooking fee. I found this part a tad confusing, and it wasn't clear whether there would be any difference if it was a space available issue vs. weather vs. mechanical etc...

At the end of the day, I will probably end up just changing the booking to the next day, e.g. to a daytime Tuesday flight, and then do an 'earlier' same-day standby for the 12:35 a.m., technically Tuesday morning, red-eye out of YYC.

-J