ssflyer
Not sure quite what you want. The fact is 428 seat aircraft have already operated EK41/2 on an ad-hoc basis (3 this week) and that is purely due to high economy loads as half-term approaches.
I make it 82 flights thus far on EK41/2 with 18 of them on 428 seat aircraft. The only strange aspect is that September saw 11 compared to 4 in August yet the August figures were much better. Also note, EK39 and 37 have seen downgrades to 360 seat aircraft at times This information can be sourced from the two links below
The first link will provide the aircraft by entering UAE42 in the flight number and the second link will show the specific aircraft config in most cases.
libhomeradar - easy collect aircraft information - Search in our contact database
The EK Source - Emirates Airline Boeing 777-300ER
I can't say at what point the decision is made to upgrade to 428 and what happens to any displaced first class pax. I think a lot of people realised first class was always going to be a tough ask and I for one is not surprised that any chance EK have to fill more at the back-end it would be taken up.
Pete