I’ve tried this with IntelliJ Ultimate 2018.3 on Windows 10 and it worked.
1. Copy/duplicate original project folder inside /IdeaProjects/ via the OS file explorer (not IntelliJ).
2. Delete it’s workspace.xml file inside .idea folder.
3. Delete its .git folder (since you’re starting a new project, based off an existing one).
4. Open the project (do not Import) via IntelliJ
5. IntelliJ might complain about a missing Git directory. Go to File > Settings, Version Control – delete anything in red, like . Click Apply & Ok.
6. Double check your Project Structure panel, make sure Modules language level is set to appropriate values for your code.