As discussed in the Magnolia user-list, there will be changes in the content structure when migrating from Magnolia 2.1.5 to Magnolia 3.X.
Magnolia Packager as part of Magnolia Enterprise Edition supports typical tasks in productive environments.

We provide a special migration license (cost: € 58 incl. tax) with which you can create a package on Magnolia 2.1.5 and install the package in Magnolia 3.X. The content will be migrated automatically. See how it works in the migration manual.

If you are interested please buy the migration license using PayPal and we will send you the necessary files.