I agree with the advice to wait a bit and see how it shakes out.
OP, a couple of further thoughts, in case this doesn't go back to being a 787:
- Your original flight has been cancelled and replaced with a new one with a different flight number. Practically, this should allow you to call Aeroplan and indicate that your flight has been cancelled and the re-booked flight is not acceptable, so you want a full refund. No need to delve in to details about why you're unhappy with the new flight and no need to worry even if the new flight is at exactly the same time as the previous flight. Historically, a cancellation like this has been essentially carte blanche to cancel/change.
- If, for some reason, point 1 is unsuccessful, you also have recourse to the "schedule irregularities" portion of the AC tariff. Rule 80(C)(1)(c) defines "Substitution of equipment or of a different class of service" as one of the schedule irregularities. Under Rule 80(C)(4)(d), you can choose no longer to travel and request an "involuntary refund", which essentially means that you get a full refund without penalty.
Note that my comments in point 2 above differ from the advice provided by YEG USER earlier. YEG USER's comments assume that you decide to travel on this ticket, in which case Rule 100(D)(2)(c) that he has cited is indeed the operating provision. However, if you invoke Rule 80(C)(4)(d) to not travel, Rule 100(D)(2)
(a) is the relevant provision regarding refunds. This states that "a full refund will be issued".
In summary:
if you prefer to fly on the 787, wait it out a bit and see if it comes back. Once they reconcile the discrepancy between the outbound and inbound being different aircraft types, you should have your answer as to which equipment will likely operate it (always subject to further change, of course).
If you prefer to fly UA regardless, just cancel now. My point 1 should be sufficient, but if not, point 2 should get you there.