I have a booking from LAX-MAD-FRA. I now need to go from DFW-MAD-FRA on the same day. DFW-MAD-FRA is available. If I book it, I would have two seats on the MAD-FRA leg (it's the same leg).

Will it let me book it? I would immediately call in to cancel the LAX-MAD-FRA trip, and hunker down for the long fight to get the avios back. (I have enough Avios to make the second booking without having to wait for the refund of avios back.) My concern is that I will have had two seats on the same flight for however long it takes me to call in and get my first ticket cancelled. Will I face consequences -- I assume they don't like that. I wish they would allow a simple change of origin, but that seems like a no-go.