Upgrades from Earlier Versions to SQL Server 2014

We can find the supported upgrades to SQL Server 2014 from the below link:

https://msdn.microsoft.com/en-us/library/ms143393.aspx

SSRS Upgrade: 

• Upgrade: You upgrade the Reporting Services components on the servers and instances where they are currently installed. This is commonly called an “in place” upgrade. In-place upgrade is not supported from one mode of Reporting Services server to another. For example, you cannot upgrade a Native Mode report server to a SharePoint mode report server. You can migrate your report items from one mode to another. For more information, see the ‘Native to SharePoint Migration’ section later in this document.
  
• Migrate: You install and configure a new SharePoint environment, copy your report items and resources to the new environment, and configure the new environment to use existing content. A lower level form of migration is to copy the Reporting Services databases, configuration files, and if you are using SharePoint mode, the SharePoint content databases.

https://msdn.microsoft.com/en-us/library/ms143747%28v=sql.120%29.aspx

Well defined steps of SSRS to new Server:

http://www.mssqltips.com/sqlservertip/2692/migrating-sql-reporting-services-to-a-new-server-by-moving-the-reporting-services-databases/

In Place Upgrade Of SQL Server:

http://sqlmag.com/sql-server-2014/migrating-sql-server-2014

Known Issues:

http://www.sanssql.com/2013/11/upgrade-error-valid-database.html