Hi John -
You are perfectly safe to reduce the size of a volume. You can't reduce the size of a volume to a point smaller than the amount of data it contains. DoT will not let you do that.
Remember when you think of size, a volume is not a contiguous nor even a specific set of blocks in the aggregate. Literally the volume is "everywhere" in the aggregate. That's part of the reason NetApp calls the underlying system WAFL - Write Anywhere File Layout. The size of a volume is only a number which says how many total blocks a volume might contain. The space guarantee (thin or think provisioning) tells you whether the needed blocks are set aside for that volume (guarantee = volume) or whether it's first come first serve and you might not get what you want (guarantee = none). Even when reserved, you aren't setting aside any sepcific blocks, just a certain number of blocks.
Since the blocks are already there, DoT will just change the maximum number of blocks associated with the volume, but will not throw away any you are currently using. Nor does any data have to be moved around or realigned simply because the size changes. This isn't like changing a partition on a disk where the data has to reside in a certain section. Go ahead and set what's appropriate for your needs.
I hope this helps you.
Lead Storage Engineer
Huron Legal | Huron Consulting Group
NCDA, NCIE - SAN Clustered, Data Protection
Kudos and accepted solutions are always appreciated.