There technically doesn’t have to be a recasting of Chadwick’s BP with the way things are set up. Since they’ve played with time travel already & now are playing with the multiverse, they set themselves up for two options.
The first option would be to bring back Michael B Jordan, who could easily have been Black Panther in another universe. People also love him & have been asking for it, so there is a fan base for that decision. It could also be a temporary solution until option 2 plays out.
The second option would be to age up Prince T’Challa II. They could do it with a time jump (which he could be at least a teenager by Avengers Secret Wars) or use time travel/multiverse (I wouldn’t like this though because it diminishes the ending of Black Panther II). They did a 5 year time jump in Endgame so it’s possible. This way the recast would be for the child but not the legacy of Chadwick Boseman. Since the kids name is also T’Challa, this also allows Marvel to continue adapting storylines from the Black Panther comic. For example, there can now be a storyline where T’Challa dates Storm. They’d just have to age him up enough to whomever they cast for Storm. I think it will be best to let Shuri take over for the next Avengers movie while having her mentor a teenaged T’Challa II so the audience starts getting familiar with the character. The audience already will have a bit of love for him just because he is Chadwick’s offspring in the movie.