If I had to guess, I would guess that James Gunn might be changing an origin for one of the Guardians of the Galaxy characters in the third movie.
James Gunn appeared recently at the E3 where he offered he is wrestling with the idea of changing something that was shown in the first movie's Nova Corps files scene (via Cinema Blend):
I’m writing Guardians of the Galaxy Vol. 3 and I finished the first draft of the treatment yesterday, which I’m excited about it and I feel really great about it. But there’s one little thing that I had in the first movie… Now, Marvel, MCU canon is crazy and I’ll get super geeky for two seconds. There’s the 616 universe which is the Marvel Comics Universe, which people get hung up on that canon, and then there’s the MCU, The Marvel Cinematic Universe, which has a separate canon which is inspired by but not exactly the same as the 616. I did something in the first movie of Guardians where it was in the background but it was distinctly there, and people know that it’s there. And I’m like goddammit because I have a really good storytelling reason for breaking the canon, and I stayed up last night figuring out if I’m gonna do it or not. I still don’t know.”
Gunn went on to explain the scene:
I’ll actually tell you. In the first movie there is the mugshot sequence where the characters go through and they’re going through the mugshots and having their picture taken and the Nova Corps has information about each of the characters coming up on their little machinery. Now, those things are something that your average viewer doesn’t see at all and doesn’t read, but there’s a lot of background I put into those things about who they’ve worked with, where they’re from, what’s going on. And it’s one of those things that I’m thinking about changing.”
While James Gunn doesn't specifically mention what he is going to change for Guardians of the Galaxy 3, if you see the images below from the Nova Corps mugshot scene, I would have to guess it's probably related to the origin or species description. It's possible Gunn may want to have a different origin for one of the characters i.e. Gamora might not be a Zehoberi or Rocket might not come from Halfworld or Groot might not be of planet X. We can further guess that the reason Gunn might want to make the change is to possibly have the character related to another character in Guardians of the Galaxy 3 (they're from the same planet or species, etc.).
I don't think it's that big of a deal if Gunn makes a change. I actually tweeted Gunn that he could simply chalk it up to the Nova Corps files not having the right information.