There have only been a few, rare instances in the Marvel Cinematic Universes where initial casting choices just didn't work out. Perhaps the biggest change came in the form of Bruce Banner, played by Edward Norton in The Incredible Hulk and subsequently portrayed by Mark Ruffalo. Because some people still think there needs to be an in-universe explanation for actor swaps, one Redditor has an interesting theory about Banner's change in appearance from The Incredible Hulk to The Avengers.
If you recall in Avengers, Banner distinctly remarks on how he tried to commit suicide. He put a bullet in his mouth, but "the other guy spit it out." Obviously, that's fairly graphic, and it did come as a bit of a shock in Joss Whedon's mostly lighthearted blockbuster. The line demands your attention, briefly lingering in a grim yet impactful way.
There's a deleted scene from The Incredible Hulk depicting what is likely the same attempted suicide Banner mentions in Avengers. Banner finds himself somewhere in the Arctic, atop a massive glacier. He pulls out a revolver and pulls back the hammer of the gun, seemingly ready to go through with the tragic act. Before he can, though, he transforms into the Hulk. Enraged, Hulk crushes the puny gun and the scene cuts back to a shot of the glacier breaking apart as the Hulk lets out a roar. It's not clear whether Banner got that shot off, but, for the sake of continuity, it's fairly safe to assume he did.
The fan theory goes that the shot to Banner's mouth caused a great deal of physical trauma, such as breaking bones and the like. When Banner transformed into the Hulk, the change caused all the wounds to heal, but they did so in a manner that made Bruce's face change to resemble Mark Ruffalo rather than Edward Norton. And thus, you have your Avengers Bruce Banner/Hulk.
As weird as it sounds, this theory isn't completely outside the realm of possibility. If a Hulk transformation were to heal wounds Banner suffered, it could certainly come with a few resulting physical hiccups. However, there's one glaring point this theory ignores. This deleted scene was meant to be an alternate opening to The Incredible Hulk. It doesn't take place between that film and Avengers. It's before both of them, in fact. If this change were legitimate, it would have been Ruffalo portraying Banner in The Incredible Hulk, not Norton.
On top of that, aside from their faces, Mark Ruffalo and Edward Norton have completely different physiques overall. Norton is leaner than Ruffalo, and Ruffalo's complexion and skin tone are noticeably deeper than Norton's. A gunshot to the head doesn't explain the rest of those physical changes. They also don't sound like one another. Sure, if the bullet went through the right parts, it could change Banner's voice, but that's a hard point to sell.
While there are some enjoyable elements to this theory, it ultimately doesn't work. It boils down to, frankly, another pointless effort to explain why there was a change in actors, something that doesn't need to be explained. You're better off reworking the canon in your mind by placing the new actor in the old one's spot. Meaning, just imagine Banner always looked like Mark Ruffalo. The "Edward Norton version," as it were, doesn't necessarily exist.
Obviously, that's not to say the Norton doesn't get the credit for The Incredible Hulk. He gives a solid performance as the character, given what he had to work with. The same can be said for characters like Rhodey/War Machine and Howard Stark (the latter of which is a bit more complicated to deal with). Terrence Howard still gets the credit for Iron Man. As for Howard Stark... Well, he got bounced around more than any other character, in terms of switching up the actors. It seems like John Slattery has a more notable claim to the role, but Dominic Cooper gives just as strong of a performance. Think of that one like shared custody, if you will.
As for Bruce Banner, Mark Ruffalo owns the role now. Until the character's time properly comes to an end, he will likely continue to do so. And while fans shouldn't completely dismiss Edward Norton's time in The Incredible Hulk, it's not worth putting stock in a flimsy theory trying to explain the change in actors. You're better off paying little mind to stuff like that and just going with the flow.