I wouldn't get to worried about Emirates swapping planes occasionally - those numbers for their non-London UK flights are amazing - we know they have a lower cost base than say BA and they don't charge THAT much less - and +75% normally means you are making a lot of money
I suspect they may swap if they can use a larger aircraft elsewhere on the day - that's the sort of flexibility you should have - not a slavish mindset that sends out the same plane everyday regardless
My question is what do they do going forward? Add more flights - maybe 10 a week rather than 7??? You'd have to be pretty brave to stick a 380 on the route but I've seen them in Aukland and they can't have many more pax than NCL, GLA or MAN