Migrating SSRS from SQL Server 2012 to 2016
Try RSMigrationTool.exe and RSMigrationUI.exe from Microsoft

Here's  a brief excerpt from the white paper link I included below, together with the migration tool download URL and Backup/Restore Encryption Key URL:


Download the tool at http://www.microsoft.com/en-us/download/details.aspx?id=29560.
Extract the files from the MigrationToo.zip file. You need the following two executable programs in the extracted files.
RSMigrationTool.exe: Command line utility that helps in exporting the reporting services artifacts. 
RSMigrationUI.exe: GUI interface that allows the users to export the reporting services artifacts. 
Export the SSRS items by using the RSMigrationTool.exe.

a.  Open a command prompt as an administrator.

b.  Navigate to the folder that contains the extracted RSMigrationTool.exe.

c.  Run the tool with the following parameters.

RSMigrationTool.exe [-n native_report_server_url] –I
native_report_server_instance [-h] –f
migration_metadata_files_folder [-p password]


White Paper

http://download.microsoft.com/download/D/2/0/D20E1C5F-72EA-4505-9F26-FEF9550EFD44/MigratingContentBetweenReportServers.docx

Migration Tool

https://www.microsoft.com/en-us/download/details.aspx?id=29560

Backup and Restore Encryption Keys

https://docs.microsoft.com/en-us/sql/reporting-services/install-windows/ssrs-encryption-keys-back-up-and-restore-encryption-keys?view=sql-server-2017

Alternately try restoring the SSRS databases

And, if you want to go that route, you can try to restore the SSRS ReportServer and ReportServerTempDB  databases to the target server. This example shows moving from SQL 2005 to 2008 SSRS:

https://docs.microsoft.com/en-us/sql/reporting-services/install-windows/ssrs-encryption-keys-back-up-and-restore-encryption-keys?view=sql-server-2017

As always, try all these things on a test/dev SQL Server first.