I think that this needs to be qualified to avoid being misinterpreted. Flights or packages booked through expedia can be changed in accordance with the fare rules, and indeed, while some OTAs tend to add their own additional change constraints, this is not the case of Expedia in my experience. Please correct me if I am wrong, but what I believe c-w-s is warning you about here is that if you do want to change your flights (or cancel them if the fare rules permit), then it will need to be done through expedia as BA will not touch the ticket directly. Moreover, expedia may charge their own change fee in addition to the ticket change fee if applicable.

So booking through expedia won't make the ticket any less changeable than it would have been if booked directly from BA but it may be a bit more of a hassle.

Also on upgrades, you can still UUA or AUP but indeed POUG does not normally appear for agent-booked reservations (that said, it does not appear for connecting flights booked directly on BA either!)