Have them converted to ProRes 422 files. And make sure however they are converting them, they retain whatever project they used to do it. If they used Resolve...then they need to keep the Resolve logs, or whatever. So you can send an EDL or XML from FCP to the colorist who can then grade the original camera masters in Resolve.
Or...if they will grade in FCP or Color...then the ProRes files will be fine. ProRes HQ if you have the space, and IF they are indeed going to go that route. But not sure you want all the rushes as ProRes HQ 4K files. You'll need serious hard drives to play that back...and a LOT of space.
It's really wise to know how you will be finishing before you do any converting. Helps figure the process out. Without knowing that, you are seriously flying blind...and can paint yourself into a corner really quick.
I was speaking of DaVinci Resolve. And sorry, I know nothing about the Phantom, or if those files can be brought into Resolve. Communicate with the DIT about conversion, and how they intend to convert, and how that can be tracked later for online, relinking to camera originals.
Get a post supervisor on this if you can. If not talk to a finishing house ASAP. All roads lead to the online, so you need to figure out how that is done, THEN figure out the path you are taking to get there.