And Iron Patriot had his face change as well. It's almost as jarring as Felix Leiter drastically changing his appearance in the older Bond movies.
Recasting is nothing new. As far back as 1935, you had brunette Valerie Hobson replacing a blonde Mae Clarke in Bride of Frankenstein. And, of course, you had John Carradine replacing Bela Lugosi as Dracula in the sequels to the original Universal film.
Well the Marvel Avenger film series' equivalent of Felix Leiter, Phil "I Watched You While You Were Sleeping" Coulson, was played by the same actor in many instalments.
I don't think recasting requires in-universe explanations. Willing suspension of disbelief and all that . . ..
I'd like to have seen George Lazenby remain as James Bond in Diamonds are Forever and through the Moore era. He could have played a vengeful 007 hunting down Blofeld in DAF and would have been still relatively young had he remained in the role until the mid-1980s. Robin Hood: Prince of Thieves with an actor who could sound English. Maybe Mel Gibson, if no actual Brit was available? Kurt Russell instead of Sam J. Jones in Flash Gordon. Alec or Adam Baldwin or maybe Michael Biehn as Batman (1989) opposite Michael Keaton's Joker. Directed by Ridley Scott or James Cameron. A Bryan Singer-directed X-Men 3. Richard Donner-directed Superman 2-4.
I admit I would be curious to see a movie that ALMOST happened: I AM LEGEND, starring Arnold Shwarzenegger, directed by Ridley Scott. That was actually in pre-production before Warner pulled the plug. Would've been interesting . . ..
George Lazenby was misled by a terrible agent who felt the Bond franchise was dying (but to be fair it was in bad shape when a surprisingly old looking Sean Connery starred in a relatively low rent looking Diamonds Are Forever). Actually Kevin Costner wanted to speak with a British accent, but for some reason the director and producers vetoed it. Things started going wrong with the X-Men movie franchise when he slunk off to direct the somewhat underwhelming (from what I've heard) Superman Returns. But The Last Stand was still better than the utterly abominable Wolverine (which had some of the worse CGI for a big budget movie ever, that looked as cheap and shoddy as the SFX used in the last two 1980s Superman sequels).
Some Hammer Films that never happened: I AM LEGEND (An early attempt at filming Matheson's novel, which was nixed by the British censors). VAMPIRELLA. The poor actress who was cast as Vampi posed for some publicity shots, which got lots of coverage in FAMOUS MONSTERS OF FILMLAND and VAMPIRELLA magazines at the time, but Hammer imploded before the movie actually started filming.