I was under the impression now that the vast majority of ATC systems would now 'hold' and display the previous code along with the emergency 75/76/77 codes?
Essentially, yes they do. The system remembers the ident of the aircraft before it switched to the 75/76/7700 code and will still display it. However, there is now the added bonus for controllers in that the emergency aircraft will show through all filters and thus those working other sectors will immediately know you're dealing with such.