Short Powershell that sets a single SharePoint Online list to a chosen look: ClassicExperience (old UI), NewExperience (modern look) or Auto Setting.
Old UI (ClassicExperience):
Modern UI (NewExperience):
In order to run the script, download, open the file and edit the 2 lines below:
Set-ExperienceOptions -Username $username -Url $Url -password $AdminPassword -ExperienceOption NewExperience
The script and several of its variations are also available on GitHub: https://github.com/PowershellScripts/SharePoint-Online-Modern-or-Classic-Experience
Don't hesitate to report issues or request new features/scenarios!