if you have more than one disk mounted in that diskgroup it is easy. Lets say you have 4 Disks with 200GB, then you can drop one of the disks out of the group. ASM will rebalance the data on the other 3 disks in the group and then release the disk. The free disk can be reused in a smaller diskgroup.
If the 800 GB are a single disk/LUN you need do add a new 600GB disk to the asm group and drop the existing one. ASM will move the data from old to new disk during the operation. Breaking up the large 800GB disk into smaller fractions would be the next step before creating a new smaller group.
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"you can use the ALTER DISGROUP statement to alter a disk group configuration. you can add,resize,or drop disks while the database remains online. Whenever possible,multiple operations in a single ALTER DISKGROUP statemetn are recommended. ASM automatically reblances when the configuration of a disk group changes. By default, the ALTER DISKGROUP statement does not wait until the operation is complete before returning. Query the V$ASM_OPERATION view to monitor the status of this operation".
Thanks Tom and Emad for your replies, very useful.