Two different databases on the same RAC cluster
We just did the same thing.


install software in new machines and configure different cluster
copy tnsnames.ora, orapwSID files to new machines and prepare pfile
you may need to add SID to /etc/oratab file on new machines
shutdown database on old machines
dismount ASM diskgroups on old machines
present LUNs to new machines
mount diskgroups on new machines
start database on new machines
configure database, instances, services using srvctl on new machines


Now you can unconfigure and remove old machines from old cluster. Of course Oracle version on new machines has to be the same as on old machines. Or it is possible to do upgrade right away on new machines.

You should test procedure of course. There are quite a few possible problems on the way.

The idea is that Oracle DB does not store anything in "cluster". All the data is in datafiles, controlfiles, redo logs and spfile. Which is stored on ASM can can be mounted on another server.