Have you checked the new redemption rates for Mileage Plan which go into effect next month March? Does AS collect carrier surcharges for the particular airline you plan to fly? Have you compared the cancellation fees charged by BA versus QR, which now also uses Avios as its redemption currency? (Avios are freely transferrable between BA, IB, and QR.)

I would expect that an award cancellation would be done pretty quickly, but your expectation that you will find two more Business Class award seats at T-331 after you have already booked two at T-355 might be unrealistic. And there is also no guarantee that the two award seats you cancel will go back into award inventory immediately -- or ever -- so do not count on that.

While just about anything is possible, if you are able to snag the two Business Class award seats at T-355, I would not count on being able to secure possibly cheaper award seats at T-331.

Good luck!