With Magnolia Packager you have a tool which supports the migration of content created in Magnolia 2.1.5 to Magnolia 3.x.

• To make the content migration please use a clean Magnolia 2.1.5 and a clean Magnolia 3.x installation
• Install Magnolia Packager on your clean Magnolia instances (2.1.5 and 3.x) (see installation)
• Export the content you want to migrate from your installation using XML-export
• Import the content to the clean Magnolia 2.1.5 installation using XML-eimport
• Create a package on the clean Magnolia 2.1.5 instance (for details see here)
  • In AdminCentral go to Packages section
  • Right-click  + New and rename the package entry
  • Right-click + Open
  • Go to Properties and click "Edit definition"
  • Go to Rules and select "website"
  • Define the rules for the documents to be included in the package using
    • "Include" and "Exclude"
    • "Selected and sub pages"
    • path for the rule (use "Choose")
  • Save the Package Definition
  • Close the package dialog
  • Right-click on the package entry + Backup and download the package file
• Upload the package to the clean 3.x instance (for details see here)
  • In AdminCentral go to Packager section
  • Right-click + Upload
  • Choose the package file created with 2.1.5 an click upload. A new package entry will be created.
  • Right-click + Open
  • Click install and wait until ready. This could take some seconds or minutes depending on the package size