Apologies for reopening a very ancient thread but I had the exact same question.

I just spoke with BA, who said that once travel commences, if one pax needs to return on another date, a change can be made to split the PNR, but definitely the mover, and in all likelihood both pax, would be downgraded to the purchased class.

What they couldn't tell me is what happens if I book this via a travel agent, as I'd prefer to do due to much better availability than relying on redemption seats.

Does anyone have any more up to date information to shed on any of this? Much obliged...