I tried it on a couple. On one, it gave me the button, but once I signed in, it said I could only change it by phone. Not terribly surprising, given this was NAP-FRA (very long stopover)-YYZ-YYC with NAP-FRA flown long ago.

The second one is a simple YYC-YVR one-way. It let me request to change it, showed me options with prices, but froze when it took me to the payment screen. The red plane wasn't even spinning, it was just stuck in one place.

Unfortunately, it also only offered me the option to change the one leg that was already booked, whereas I wanted to both change that and add a return. So it seems even when they do get this working, it may have pretty limited functionality. But, I guess if it takes some pressure off the call centre by allowing some number of things to get done online, it will be better than nothing.