It doesn't appear as though Robern Downy Jr. or Chris Evans are getting recast. They simply walked off into the sunset when their story was done... something that can happen in the movies, unlike the comics. I suspect that's the fate for pretty much all the original Avengers. They all pretty much have legacy characters lines up to take the mantle moving foreward.
Which of course doesn't necessarily mean that's going to happen to T'Challa... YET. Again, that's obviously up to Coogler. If he's got more T'Challa story to tell, then he'll recast T'Challa. If he's decided to move on and more foreward with Shuri, he can do that too. Unlike the comics, the MCU movies seemingly will do well no matter what, so there's less risk of actual change if/when the movies are are ready to go that route.