Originally Posted by
noobynoob
I was using schedule irregularity (which I gather you referred to as irregular operations) although it may not squarely fit in b (original yyz-bos either cancelled or changed flight number and time) or c (new flight has a different aircraft that the one I booked) - Some of the terms I find gray are (apparently) crystal clear to the experienced folks here.
Definition “Schedule irregularity” means any of the following:
a) Delay in scheduled departure or arrival of a carrier's flight
b) Flight cancellation, omission of a scheduled stop, or any other delay or interruption in the scheduled operation of a carrier's flight, or
c) Substitution of equipment or of a different class of service, or
d) Schedule changes which require rerouting of passenger at departure time of the original flight.
Anyway, I guess my options are either rebook or a full refund. There doesn't seem to be too many options to change given the holiday season and limited flight availability.
I just ate lunch while reading the International Tariffs. Thanks
noobynoob 
For those who want to read along, AC's international tariffs are here:
https://www.aircanada.com/content/da..._Tariff_en.pdf
The relevant section is on page 93, RULE 80 – SCHEDULE IRREGULARITIES
The very first part of that section states
Schedules not guaranteed. Times and aircraft type shown in timetables or elsewhere are approximate and not guaranteed, and form no part of the contract of carriage. Schedules are subject to change without notice.
and that sets the tone
For the record, I think that AC is using the term "Schedule irregularity" as a broad term meaning everything from schedule changes, right the way through to irregular operations (irops).
The tariffs are broad (and very much in AC's favor) but I think they are reasonably clear.
Rule 80.C.4
In the event of a scheduled irregularity, carrier will either:
a) Carry the passenger on another of its passenger aircraft or class of service on which space is available without additional charge regardless of the class of service; or, at carrier's option;
b) Endorse to another air carrier with which Air Canada has an agreement for such transportation, the unused portion of the ticket for purposes of rerouting; or at carrier's option;
c) Reroute the passenger to the destination named on the ticket or applicable portion thereof by its own or other transportation services; and if the fare for the revised routing or class of service is higher than the refund value of the ticket or applicable
portion thereof as determined from RULE 100 - REFUNDS, carrier will require no additional payment from the passenger but will refund the difference if it is lower, or,
d) If the passenger chooses to no longer travel or if carrier is unable to perform the option stated in (a) (b) or (c) above within a reasonable amount time, make involuntary refund or general refund as applicable in accordance with RULE 100 - REFUNDS (an exception to the applicability of a refund occurs where the passenger was notified of the schedule irregularity prior to the day of departure and the schedule irregularity is of 60 minutes or less) or,
e) Upon request, for cancellations within Air Canada's control, return passenger to point of origin and refund in accordance with RULE 100 - REFUNDS as if no portion of the trip had been made (irrespective of applicable fare rules), or subject to passenger's agreement, offer a travel voucher for future travel in the same amount; or, upon passenger request.
f) For cancellations within Air Canada's control, if passenger provides credible verbal assurance to Air Canada of certain circumstances that require his/her arrival at destination earlier than options set out in subparagraph (a) above, or, for on my way customers, for cancellations within or outside carrier's control, Air Canada will, if it is reasonable to do so, taking all circumstances known to it into account, and subject to availability, buy passenger a seat on another carrier whose flight is schedule to arrive appreciably earlier than the options proposed in (a) above.