SQL Server supports upgrade of the Database Engine and Analysis Services from SQL Server 2005, SQL Server 2008, SQL Server 2008 R2, and SQL Server 2012 failover clusters separately on all failover cluster nodes.
Support details are as follows:
Upgrade is supported both through the user interface and from the command prompt. For more information, see Upgrade a SQL Server Failover Cluster Instance (Setup) and Install SQL Server 2014 from the Command Prompt.
Upgrading from SQL Server 2008 R2 — You can run upgrade from the command prompt on each failover cluster node, or by using the Setup UI to upgrade each cluster node. If Full-text search and Replication features do not exist on the instance being upgraded, they will be installed automatically with no option to omit them.
Service pack installation - you must apply SQL Server service packs and patches to SQL Server 2012 failover clusters separately on all nodes.
The following scenarios are not supported:
You cannot migrate from a stand-alone instance of SQL Server to a failover cluster.
Add features to a failover cluster. For example, you cannot add the Database Engine to an existing Analysis Services-only failover cluster.
You cannot downgrade a failover cluster node to a stand-alone instance.
For more information, see AlwaysOn Failover Cluster Instances (SQL Server).
You cannot directly upgrade a non–multi-subnet SQL Server failover cluster to a SQL Server multi-subnet failover cluster. For more information, see Upgrade a SQL Server Failover Cluster Instance (Setup).