Ok, just had a brainwave (well actually I was nosing around on AskTom) that uses transportable tablespaces...

- generate a transportable tablespace set ( set tablespaces to read only and perform a special export)
- shutdown database instances on both nodes
- move/rename the old system/sysaux/undo datafiles
- Create a new empty database with just the bare minimum i.e. system/sysaux/undo etc
- Import the tablespace set
- Set tablespaces in read/write

Might have to create small undo datafiles (due to mountpoint constraints) and resize them up after we drop the old ones, but at first glance I think it would work.

Anyone spot any flaws?