It may sound strange but I did this once on an AA flight, the supplement I needed to pay for the higher fare class was irrelevant seen the extra mileage I would earn and it ensured I was platinum after the outbound part of the trip.

I don't know if it's something you can do on BA.

I have a longhaul flight booked in fare class 'O'. 25% miles.
Fare class 'S' would give me 50% of the mileage and is just one class up I think? Getting these extra miles would mean I would have enough for a redemption I'm planning. But I don't want to break the bank, I don't know how much the fare difference between 'O' and 'S' would typically be.

Did anybody do this on BA before? It took a while to explain everything to the AA call center the previous time I did this, I don't know if BA is used to these type of requests...