Hey,
before i did a fully reinstall of my PC i saved the whole “Unreal Engine Editor” folder.
After the newinstall i copied the whole folder back to my new installed PC and startet the UE4.exe.
I was able to start the Editor and create projects BUT it still wants me to download the whole 3GB editor!
You did not copy the meta-data, which lives in C:\ProgramData\Epic and so the engine thinks it’s incomplete.
The transplant might only work if you copy exactly to the same paths all the folders (main Unreal Engine and meta-data ProgramData\Epic). Also, you might need to copy the registry key or two.