This is useful when we have only Managed solution to work with but needs some more customization. Extract the Solution Zip file, Open
solution.xml file in notepad application. Search for Managed Tag (E.g.: <Managed>1</Managed>), Now
change the value from 1 to 0 and zip the files again and import it. That’s it The Solution is changed to Unmanaged. This can be imported and customized as per the need.
Did this actually work for you or are you simply speculating? All other resources confirm my own experience - i.e. that this is not possible as managed solutions use a different XML schema and therefore flipping this flag is insufficient.