Yes, Robert Downey Jr. IS Iron Man, but you know the whole multiverse thing has really thrown a lot of that out of whack. In fact, there was talk for a while about RDJ being "replaced" in the most recent Dr. Strange movie by Tom Cruise. It's not secret that Robert Downey Jr. was NOT the MCU's first pick to play the Iron Avenger. Nope, they wanted Tom Cruise, but you know, Hollywood and that never happened.
Now it seems they are looking for an actor in their twenties or thirties to play a younger Tony Stark from another universe in the next two upcoming Avengers movies, "Avengers: The Kang Dynasty" and "Avengers: Secret Wars". Talk is the guy that played Eddie in "Stranger Things" may be up for the role, but it will be interesting for sure and you can see more here