We fans love Robert Downey Jr. as Iron Man. He is Iron Man no one could replace him! After the events in Iron Man 3, he can still be Iron Man. He is Tony Stark and he can rebuild his Armour and Arc Reactor. He is one of the main Avengers so you couldn't get rid of him.
If Robert Downey Jr. cannot be Iron Man for any reason, then in my personal opinion, they should kill off his character and let him die as a hero rather than recast Tony Stark in Avengers 2 or 3. Fans will be disappointed by a recast. Maybe in the future installments, years after Avengers they may decide to reboot the Iron Man franchise and then a new actor will most likely take the role of Tony Stark.
As much as I love Robert Downey Jr. as Iron Man and don't want another actor replace his role as long as Iron Man Movies continue to be released, but don't want to be disappointed in the Marvel Cinematic Universe franchise like any Marvel fan would also.
What do you think should happen if this does become the case one day - hopefully it doesn't!