I assume I'd just pop the SQL Server 2012 disc in one of the nodes and install a new SQL Server Cluster instance and then go around to the rest of the nodes and just use the 'add node' option, but I just wanted to verify first.
That's exactly how you do it.
Because you now have two instances and may want to run them on separate nodes, you should adjust the preferred owner list in cluster manager. Also remember to adjust the max memory settings, in case you have to run both instances on one node.
[font="Verdana"]Markus Bohse[/font]