in-place upgrade native SSRS without the SQL Server portion We have a single server (2012R2) running SQL 2008R2 + reporting services native mode 2008R2
Can the reporting services portion be upgraded alone without the database engine?
Or can the SSRS db be migrated off, SQL instance shut down and then SSRS alone upgraded?
Because of licensing concerns I cannot have SQL Server 2016 running but I still need to upgrade the SSRS to 2016.

They require the same license.  So if you can't run SQL Server 2016, you can't run SSRS 2016.