It almost certainly won’t be a straight swap like that even if BA order more aircraft and as now you will see a mix of 772, 77W, 787 and A380 as well as the forthcoming A350 depending on route and demand. However some have suggested that there may be a concentration of the 772s on these routes (especially if they are all refurbished and densified) as they are older and less fuel efficient, however it is all to some extent crystal ball gazing.