Steam players can head over to %localappdata%\Pal\Saved\SaveGames to find their save files. If you’re on Microsoft Store, use %localappdata%\Packages\PocketpairInc ...
Next-gen game upgrades never seem to go as flawlessly as you'd hope, so I was not at all surprised to find some potholes to avoid in transferring your GTA 5 save file to GTA 5 Enhanced edition on PC.