EK17/18 will offer First (private suites), business and Economy, EK19/20 will still offer just Business and economy. No upgrade to a 3 class B777 as suggested, which is very sensible in my opinion.
I would agree that it is a sensible decision in terms of passenger numbers as swapping the evening service for a 3 class aircraft would end up seeing an overall reduction in capacity which would obviously then mean a 3rd daily needed.
However, do EK operate any other route with a mixed service, as in one flight is 3 class and one is 2 etc? I can't think of any and for that reason I think it will change and an A330 (or A343 as mentioned) could end up filling in the gap in capacity.
If there are any destinations that have two or more flights with different cabins (F/J/Y v J/Y) then I apologise as I cannot think of any.