He explained upthread that by purchasing the RT ticket originating in BLL it was significantly less expensive than just changing the return date of his first ticket by one day. RT BLL-USA-BLL with M outbound could easily be much less than changing the return date of a US-originating ticket to M class, which incurs both a change fee and an upsell to M class.
It is very logical that it could save this much money. Remember, round trip fares originating in the USA to BLL are not the same as round trip fares from BLL to the USA (or to/from any international city for that matter).
Thank you. I somehow missed the logic that one of the tickets would now originate in the EU rather than the USA.