Upgrade older SQL 2008 to 2008 R2 on active/passive cluster
It shouldn't be that complex.

On the passive node upgrade the instance to SQL 2008 R2.  Then reboot that node.  Then failover.  At this point the database will be upgraded to SQL 2008 R2.

On the now passive node upgrade the instance to SQL 2008 R2.

There's an article I wrote about upgrading a SQL 2005 cluster to SQL 2008.  It's basically the same process.