1. I think I can add higher sized disk into spare and when one of the lower sized disks fails , it will still get replaced with the higher ones. Correct me if I am wrong. But the documentation says its possible without disturbing the RAID structure. Only drawback is that the extra space will get wasted. Thats ok with me. But please confirm that its possible.
yes
2. Secondly I do have same sized disks as well but they are not new. They are taken out from older Netapp which is not in use anymore. So first , am I allowed to add such disks straightaway as spare in existing Netapp? If yes , will the procedure be same? Normally we just take out the failed disk and insert a new one in the same slot. And using command <disk assign disk-id> I would add the new disk to spare. Will this procedure remain same or I should be deleting existing data from the disk first? I assume there could be data on these disks, as they were in use in older Netapp. Kindly point me in right direction.
you should unowne the disk if possible in the old system
options disk.auto_assign off
priv set advanced
disk remove_ownership disk_name
disks should unowned
remove disks
options disk.auto_assign on
or take onwership on new system
priv set advanced
disk reassign -s -d <new sysid>