Great timing for your post. I found the answer about a week ago but did not have time to post it until yesterday.
It is post #11
Schedule changes/cancellation
I provide some more details in that post, however, this is the new reference:
If someone at AC is not aware of CIC*77 details can be found in ACpedia (ACP) under "Schedule Change (SKCH) General Policy and Handling Guidelines".
I noticed that your routing is
from LHR. I believe the European Passenger Bill of Rights may also entitle you to some additional benefits.
I know you were considering a routing through LHR however you could ask to fly BA (OAL - Other Airline) if AC will not provide an acceptable routing. Don't forget, if you decide to leave the next day, AC is to pay for your hotel.
Let us know what happens. I am interested to know if the agent realized what you were entitled to before you mention CIC*77 or the new ACP (ACpedia) reference.
Cheers