1. offline the datafileHere, RMAN assigns automatically a unique name for the new datafile (OMF) and you only have to specify the new diskgroup name while using the COPY command which makes things easier. But say if you are using ASMCMD, then you cannot just specify the new diskgroup name for the CP command, which means that a unique name cannot be obtained automatically.
2. RMAN>copy datafile <offline-datafile-on-old-DG> to +NEW-DG;
3. At database level, rename the old datafile to the new name reported from the above command
4. Switch the new datafile to copy
5. Recover the new datafile
asmcmd>cp OLD_DG/datafile/abc.123 NEW_DGYou need to specify the destination name as well something like below:
asmcmd>cp OLD_DG/datafile/abc.123 NEW_DG/abc.dbfHere you are getting a normal file and not OMF, though ASM internally creates a link for the file "+NEW_DG/abc.dbf" pointing to an OMF. ASMCMD makes you a bit more confusive.