To add new disk array / lun

sqlplus / as sysasm
ALTER DISKGROUP DATA ADD DISK '/dev/rdsk/c1d1s0' REBALANCE POWER 5;

# Fastest 
ALTER DISKGROUP DATA ADD DISK '/dev/rdsk/c1d1s0' REBALANCE POWER 11;

To verify

SQL> select path,name,header_status from v$asm_disk;

To Check where the rebalnce is up to

select INST_ID, OPERATION, STATE, POWER, SOFAR, EST_WORK, EST_RATE, EST_MINUTES from GV$ASM_OPERATION where GROUP_NUMBER=1;

To remove the old one

# Use the name showing in select disk name
alter diskgroup data drop disk DATA_0000 rebalance power 5;