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
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