The size is about the same. Fortunately.
For unpacking I used UnrealPak which is part of the UE4 installation and can be found here:
There's obviously some other tool I just happened to find but didn't try.
Unreal Engine 4 pak-file Unpacker / unreal_tournament_4.bms [UE4 global topic] - ZenHAX
If needed I'll upload the packer I used. Just let me know.
It's a command line tool and lets you unpack with the following command:
UnrealPak.exe CyberdimensionNeptunia4GoddessesOnline-WindowsNoEditor.pak -extract x:\your\path
You will get two folders:
Merge each with the already existing one. Maybe make a backup of your game folder first because as we all know, shit happens.
Invalidate or delete the PAK. Not sure if overriding actually works. I didn't try.
The uassets themselves can be extracted with the newest version of umodel by Gildor.
Just make sure you set it to UE4 - 4.13. That version is sure to work.
I haven't tried any other modding tools yet. I didn't really want to bother since this is the first Idea Factory game using UE4. So I didn't really see a point and kept trying with the engine itself.
But it's been a while I actually modded a Nep game.
If there's tools that might or still do work I'd like to know of course.
May the pudding be with you.