patch testing following a recent tutorial from magicbluemonkeydog on using original / legit PS4 game saves with FPKG's (Fake PKG games) with details below.
To quote from magicbluemonkeydog via Reddit: You CAN Use Legit Saves with FPKG Fames Tutorial
1. Dump your original game and package normally BUT make sure to select the option to allow the pkg to be modifiable.
2. Write the original keystone from your dump into the pkg like so:
3. Load up your game and rejoice as your saves work with the newly installed fpkg.Code:orbis-pub-cmd.exe pkg_file_add --passcode 00000000000000000000000000000000 --force G:\Games\PS4\CUSA00074-app\sce_sys\keystone sce_sys/keystone G:\Games\PS4\EP0001-CUSA00074_00-CHILDOFLIGHT0001-A0100-V0103.pkg
You'll want to write the keystone from your disc dump into the Complete Edition fpkg then. If the CE version you have isn't modifiable, just extract it and rebuild it with the modify option. Depending on the CUSA version, you might need to edit your save database as well, there's a post in this subreddit detailing how to do that.
I'd use the PS4 Package Viewer by LMAN to extract the complete edition. From there, it's just like building an fpkg from a regular dump, but with the mod_pkg flag set (it's an option when pressing build).
Ask someone nicely who has the original keystone file to upload it for you. Or borrow/rent the disc. The legit, non fake PKG file will still be on your PS4 if you haven't deleted the game install but without the passcode you won't be able to extract the keystone file from it.
Updates don't have keystones, at least not the ones I've looked at. Makes sense, you wouldn't want to change the keystone and invalidate legitimate saves when updating. You can use pkg viewer to check though.
Every PS4 game has a keystone file which is generated based off of the passcode used when creating the game package. This keystone file is used to identify a save as belonging to that game. As fpkgs use all 0's for the passcode, the keystone file generated when you make the fpkg differs from the keystone file in the legit game. Therefore if you try to load a legit save on an fpkg installed game it won't recognise it and will say your save is corrupt.
Yeah if the save games are already marked as corrupted, even putting the correct keystone in place won't fix it as the flag has been set. Changing the flag back in the save database, or restoring the saves/db via various other means will sort it.
The command will finish with a bunch of text and no errors, then test your game and see if loads your saves. I highly recommend that you take a backup of your save database so you can restore it if it doesn't work and your save files get marked as corrupt.
Use the PKG Viewer by LMAN, extract the fpkg you have, and then build a new fpkg from your extracted directory with the modifiable flag set. Then do the command line step to replace the keystone. There are plenty of tutorials floating around for building an fpkg from a game dump.