Our user guide contains the following section:
If you want to move off of the Jackrabbit JSR-170 repository to the file system, or to the Jackrabbit repository from the file system, or to any of the other repositories supported by the documents and media library, you can do so very easily. Make sure you have already set up your portal-ext.properties file so the hook is properly configured before running this migration. Select the Document Library hook that represents where you want your documents migrated and click Execute. Your documents will be migrated to the new repository. You can then shut down Liferay, make the new repository the default in the portal-ext.properties file and then restart.
This should be extended with the following info: https://issues.liferay.com/browse/LPS-36609?focusedCommentId=513074&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-513074