Outatowner - You are right in how this is another fine example of EK incompetence but the callsign that is used, doesn't have to have any similarity to the flight number on a ticket. In fact some operators have totally random digits for some callsigns which bear no resemblance to the associated flight number whatsoever. The EK flight numbers don't need to change. The only time I have seen a suffix added is when there has been significant delay of some kind and the aircraft goes back to DXB with a 'D' being added on to the callsign.
QCM - I am not a controller but will answer your question. The type check on first contact is done to ensure that the controller can apply the correct wake vortex separation. Take an example where an airline is operating a flight with an A320 and at the last minute there is a tech problem and an A330 is used instead. In amongst the aircraft swap, someone somewhere forgets to change the flight plan. Wind the clock forward and the A330 descends towards a busy airport but the information from the flight plan, which is available to the approach controller, says it's an A320. So separation behind a heavy needs to be applied here but the controller thinks it's a medium instead. It could mean the controller inadvertently positions someone closer behind the A330 than they should be.
The above scenario may seem a little far fetched but it is entirely possible. There is more scope for getting types wrong in other parts of the world but with minimum separation being used most of the time in DXB, it's in the controller's interest to verify what type he or she is dealing with.