The premise of your statement is incorrect. Cutting out a segment, such as changing LHR-YVR-YYC to LHR-YYC, or even LGA-YYZ-YYC to EWR-YYC, is possible.

Whether it's possible to add a segment though, I don't know.

Whoever told you that was wrong. I have done exactly those things I listed as examples. There are many other examples on this board of people having used SDC to cut out a segment as well.

That's blatantly incorrect.

The canonical example of this is "Aeroplan only let me book YVR-YYC-YWG-YOW-YYZ, and I SDC'd to YVR-YYZ". That's always permitted, and I've done things like that more times than I can count.

I have added a segment in the past, but it is difficult. One of the benefits of AC SDC (compared to, say, UA) is that they don't reticket anything. It's just a change in the AC PNR. This means they can do more interesting things, like SDC codeshares and foreign tickets, as long as it's operated by them.

However, it also means the number of coupons doesn't change. So if you have SFO-YYZ booked, you have one coupon. If you SDC to SFO-YVR-YYZ (I did that once for a pod), you end up with COUPON REQUIRED and need a FIM.

I've been told "no" more than I've been told "yes", but I've definitely done it.

I told them I wanted a pod for the transcon, and was willing to pay the fee.

They did it.

Them everything hit the fan. SFO concierge was very concerned I had no coupon. YVR called me as I was boarding to ask if I had a FIM. I said no. They said SFO should have dealt with it. They met me upon landing with a FIM. It was necessary when I boarded.

It's possible, but I'm not sure it's allowed, because it really complicates things and causes extra work.

I'm far from an expert, and usually lurk here rather than post.

However, last spring I did a SDC adding a segment. I had YYZ-YQY booked direct in lowest J. My meetings ended early, so I got to the airport early and changed to YYZ-YHZ-YQY.

My reasoning wasn't a segment run, but the fact I'd be home about 3 hours or so earlier. In fact, I got home before my original flight left.

Perhaps playing the "get home to the family" card worked. Perhaps it was that the agent had a relative from my hometown. Perhaps, it was because I was polite. Perhaps, it was because my scheduled flight was the next one to YQY, but would have required like 7 hours at YYZ, when the connector was available. Perhaps, it's totally fine.

Regardless, I've done it. I just don't know if it was because it's legal or other customer service factors.

Lots of fares have free SDC, so it isn't necessarily making them money, and it can cause a lot of hassle that would exceed even $75.

Passenger handling fees? Sure, the $75 almost certainly would cover any incremental AIFs, and other per capita fees assessed by the connecting airports, but its not literally just $75 in AC's pocket.

But seriously, they just pull the connecting flight coupons if moved to the non-stop, but if moving from a non-stop to a connecting service, they have to generate new coupons. Lots of staff don't know how to deal with this properly, so they use some sort of authority/override in their system to spin up what amounts to an unrestricted Y ticket -- the 'payment' for which gets reconciled in the back office. The passenger due to the Y ticket gets an un-paid-for LT07 entitlement or even an op-upgrade if they have status. Etc. Its easier just to discourage the practice or for staff to not do it, than to do it properly. Not that many CSR staff actually know how to do many tasks relating to ticketing because there's almost no training investment made by AC and on-the-job learning opportunities are usually clouded in extreme chaos like IRROPs. The disciplinary culture comes down heavily on mistakes made that might cause passengers to have their entitlements elevated beyond what they paid for -- something that most AC staff don't want to deal with the hassle of either. And last but not least, maybe the passenger is doing some sort of hidden city scam if they SDC a non-stop to a connecting, or otherwise deriving additional value from the connection that they haven't paid for (ie: having a brief business meeting at the intermediate city!).

I have the exact same question but my dilemna is as follows: I have all my flights scheduled for the remainder of the year (LGA-YUL-LGA) every weekend until the end of the year, and will be missing only 1 segment for 50K, is it possible to change one of the LGA-YUL to a LGA-YYZ-YUL flight (as in adding a segment so I'm able to hit the 50k) ?
Could this be possible at the gate or counter at the airport? I am usually able to get on an earlier flight very easily when I present myself at LGA as the staff there is usually nice, but adding a segment is something I've never tried and could really help me.