Ok, now that we have resolution I'll give all the details and will hopefully make things clear for everyone.
First the resolution: I moved the return flight in Y to a later date. I will not fly it, but that way it does not conflict with the O ticket anymore.
Cost me about 200 € but that's pretty cheap for a J upgrade that's not costing me any miles.
As for the what I was trying to explain: per the mileage calculator on FB's own website, you will in find that it does in fact cost the same exact number of miles to fly USA-Asia than it does to fly Europe-Asia.
USA-Asia always routes via CDG on AF. If you book a one-way USA-Asia then you can't stop in CDG, it has to be consecutive flights (that's the one catch with FB one-way awards). But if you buy a full return trip USA-Asia-USA then you are allowed one stopover either on the way there or on the way back.
So in my case:
- I have to fly CDG-USA-CDG so bought a ticket in Y - that's trip number 1.
- About a month after trip 1 is over I have to fly CDG-Asia-CDG so redeemed an O award - that's trip number 2.
- I then realized I could have flown the return of trip number 1 in O had I booked it as the first leg of an USA-Asia award with a stop-over in CDG on the outbound. So I called up FB and asked them to add the USA-CDG and CDG-USA legs to the existing trip 2 (I could not have only added the USA-CDG leg as the itinerary has to be a round trip for the stopover in CDG to be legal).
That all repriced for the exact same number of miles; all I had to pay were some extra taxes (because the itinerary now has more airports in it). Let's call that new award trip number 2+.
At which point the paid return of trip 1 was on the same flight as the first leg of the outbound of trip 2+, hence the conflict that started this whole thread. Conflict that I then eliminated by moving the return leg of trip 1 to a later date.
The more I explain it the more confusing it gets I think. But the bottom line really is what I have bolded.
Any questions let me know.