I've never tried this on the AS website but if it is, it will error out at some point. This is for all travel agents, not only OTAs, and for all airlines. If you book through and agency, you make the changes through that agency, at least until the day of travel.

In general I do not book through a TA unless I am getting a significant discount, or I have a complicated itinerary that I can't price out on the website.