Originally Posted by Excelsior.
They already have sufficient practice not noticing the sudden huge differences in Bruce Banner and James Rhodes. I think the MCU is quite ready for a different Tony by now.
Ready? No, probably it will never be "ready." Downey, more than any actor save perhaps Jackman as Wolverine and Reeve as Superman, defined his character for an entire generation with his performance. Audiences will take it much harder than ANY single character in the MCU being recast.
However, Downey will one day want to quit, though I suspect he'll at least do Avengers 2 before then. When that happens, Marvel is not writing off the character.