Method which Grindelwald used to impersonate Graves wasn't something as pedestrian as polyjuice or transfiguration, he used a complicated and powerful piece of magic to swap their bodies, so rare most wizards wouldn't even believe it possible. So now Graves is stuck as a bleached pineapple and Grindelwald is committing crimes with his body and what's worse - he got his damn self arrested and now everybody thinks Graves is psychotic dark wizard.

I can see it going several ways

1. Grindelgraves isn't resistant to veritaserum so when he spills the beans "I am actually Grindelwald, I borrowed your director's body while he is in mine" they think a. Graves totally insane b. Grindelwald put some strong magical whammy on poor Graves c. they believe in the bodyswap, rescue Graves-in-Grindelwald and things are very awkward while they try to figure out how to swap them back

2. MACUSA/ICW orders Grindelgraves execution. Graves, who has gotten out has to break him out to save him (he doesn't want to stay bleached pineapple forever)

Bonuses:
- Grindelwald doesn't want to switch back, he wants to stay Colin Farrell
- Dumbledore helped develop the spell so they need his help to reverse it ( I just want to see what he would make of the whole situation)