It is worth noting that ALL MCU movies undergo reshoots. In fact, Feige has talked in the past about how they plan them in advance, and even factor them into everyone's contracts. After all we saw Natalie Portman and Chris Hemsworth doing reshoots for Thor 4 just the other day and no one seems concerned about it.
The issue here seems to be the length of time they're allocating to this. It appears that this is less about fixing/adding a few scenes, and more like remaking a decent chunk of the movie.