If you’re more comfortable with GUI, all tasks here can be accomplished using the Enterprise Manager.

1. Add new disks to ASM diskgroups.

ALTER DISKGROUP PMDW_DG1 ADD DISK
  '/dev/rdsk/c4t60060E80056FB30000006FB300000823d0s6' NAME PMDW_DG1_0003,
  '/dev/rdsk/c4t60060E80056FB30000006FB300000826d0s6' NAME PMDW_DG1_0004,
  '/dev/rdsk/c4t60060E80056FB30000006FB300000829d0s6' NAME PMDW_DG1_0005
REBALANCE POWER 11;

We go with the rebalance power of 11 which is full throttle because it is planned maintenance.

2. Check rebalance status from Enterprise Manager or v$ASM_OPERATION.

3. When rebalance completes, drop the old disks.

ALTER DISKGROUP PMDW_DG1 DROP DISK
   PMDW_DG1_0000,
   PMDW_DG1_0001,
   PMDW_DG1_0002
REBALANCE POWER 11;