Before the movie, Credence and Original Graves knew each other. Whatever reason but preferably no Grindelwald follower!Graves or established relationship. Because of this relationship, among other reasons, Grindelwald swaps with Graves before the movie or at the very beginning.
Movie goes on as normal and then canon divergence - Credence finds out that Grindelwald has swapped with Graves. He's confused until he realises that means Graves has been hurt and captured and is being held somewhere by Grindelwald and he's not having that.
Cue Obscurus and no amount of persuasion from Grindelwald can get him to calm down. Credence rescues Graves and Grindelwald flees.
Tl:dr - Credence finds out Graves is really Grindelwald and wants the real Graves back.
Bonus: After everything Tina is the only one Credence will allow near Graves while he's recovering. Double bonus: Newt is bemused but just accepts it as New York being very strange.
Credence/Original Graves - Credence finds out about the switch
Movie goes on as normal and then canon divergence - Credence finds out that Grindelwald has swapped with Graves. He's confused until he realises that means Graves has been hurt and captured and is being held somewhere by Grindelwald and he's not having that.
Cue Obscurus and no amount of persuasion from Grindelwald can get him to calm down. Credence rescues Graves and Grindelwald flees.
Tl:dr - Credence finds out Graves is really Grindelwald and wants the real Graves back.
Bonus: After everything Tina is the only one Credence will allow near Graves while he's recovering.
Double bonus: Newt is bemused but just accepts it as New York being very strange.