What I said looks feasible .... you already have a mirror on oradata12 volume .... where first plex (oradata9-01) resides with c1t2d0 & second plex (oradata12-01) resides on c1t5d0 ...
So here would be the precise steps:
-- Make sure SAN disk is visible in VxVM, also it should have size atleast equivalent to your volume size
# vxdisk list
-- If SAN disk is not initialized with VxVM , initialize it...
# vxdisksetup -i <SAN_disk> (default format would be cdsdisk)
-- Add the SAN disk to diskgroup
# vxdg -g testdg adddisk testdg03=<SAN_disk>
-- Mirror the oradata12 volume
# vxassist -g testdg mirror oradata12 <SAN_disk>
-- check that sync in completed
# vxtask list
-- Verify from vxprint , you should see a new plex added to oradata12 volume...
# vxprint -qthg testdg oradata12
-- If above looks good, then go ahead & detach old plex
# vxmend -g testdg off oradata9-01
# vxplex -g testdg dis oradata9-01
-- If you see vxprint command now, you should see plex oradata9-01 is disassociated... If looks OK, go & delete the plex
# vxedit -g testdg -rf rm oradata9-1
-- Repeat the previous two steps of another plex oradata12-01
One more thing to notice, I see you have added a log plex, what was this log plex added for ? If it was DCO logs for snapshots, it would be advisable to move that also to SAN disk, also recommended to keep a mirror copy of log plex as well...
Hope this helps..