Can you upgrade a single SQL Server 2008 Failover Cluster Instance to SQL Server 2012? Or do you have to upgrade every FCI on the Failover Cluster? The documentation doesn't seem to be clear.

We have a 7 instances on a 3 node cluster. We want to upgrade only one of the instances from SQL Server 2008 to SQL Server 2012. Can we perform an upgrade or do we need to backup the instance as best we can, uninstall that instance, install SQL Server 2012 then add that instance back?

I know you can run SQL Server 2008 and SQL Server 2012 on the same failover cluster...

The installation makes it seem like you can upgrade a single instance, but I'm still apprehensive since we haven't tested and no documentation seems to explicitly state it's possible.
This is possible and we successfully completed an upgrade of a single failover cluster instance to SQL 2012 on a cluster with multiple SQL 2008 failover cluster instances.

Perform the upgrade of the instance on the two nodes not hosting the instance first (be sure to select the correct instance when asked as in the screenshot), then finally perform the upgrade on the node hosting the instance.