5H00T4: I'm sure a script/batch file would have been quicker in the end, but my mental capacity was just enough to manually copy and rename everything. :D
Here's a quick PowerShell script to do it for you. You can uncomment the [-whatif] in the Copy-Item command at the bottom to test it first. Be aware this simply copies the files, you should remove files from the GOG save folder first.
==========Start Script==========
#You may need to change this if your Epic save files aren't located under your profile
$EpicSaves = "$($env:LOCALAPPDATA)\Remedy\Control\Default-Epic-User"
#You may need to change this if your GOG save files aren't located under your profile
$GogSaves = "$($env:USERPROFILE)\Documents\My Games\Control\Saves"
$Slots = Get-ChildItem -Path $EpicSaves -Recurse | where{$_.name -match "savegame"}
ForEach($Slot in $Slots)
{
ForEach($Chunk in (Get-ChildItem -path $Slot.FullName | where{$_.name -notmatch "^--"}))
{
$RenamedFile = $null
$RenamedFile = ($Slot.name +"_" + $Chunk.name.split(".")[0])
Copy-Item $Chunk.FullName -Destination (Join-Path -Path $GogSaves -ChildPath $RenamedFile) #-whatif
}
}
==========End Script==========
Note: Seems this removes my formatting, script will still work, but visual formatting for readability seems to have been removed.