There is a Step by Step Procedure which needs to be followed for Migration of Reporting Services 2005 to Reporting Services 2008 or Later Version.
You Can Upgrade the SQL Reporting Services from 2005 to 2008. But, Make sure you take the RSkey backup before you do so and also there are certain deprecated functions in SSRS 2008 make sure those functunalities are Not used.
1. Backup the encryption key
2. Backup the RS databases
3. Transfer the key and database backups to the new 2008 server
4. Turn off Reporting Services on the 2008 server
5. Restore the RS databases on the 2008 server
6. Start the RS configuration tool and turn on Reporting Services
7. Reset the database connection using the configuration tool
8. Restore the RS encryption key
9. Open a new query to the ReportServer database on the old and new server.
10. On the old server, run 'select * from ReportServer.dbo.Keys' and make note of the InstallationId value for the non-null record (this record will also have the server's name in the MachineName field).
11. On the new server, run 'select * from ReportServer.dbo.Keys' and you should see 3 records. One null record, and 2 records that have values in the MachineName field (these should be the old and new server names). The InstallationId value from step #10 should be in there with the old server's name.
12. On the new server, delete the record that matches the old server's InstallationId.
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