Hi JHaynes,  It makes no sense to me to change a Thick LUN (with 1.4GB of used space) for a 1.4GB Thin LUN, because you end up with no free space left on your Thin LUN. Or do you also want to make use of thin virtual disks?  If better utilization of the total storage capacity is your goal, you might create a 1.8GB thin provisioned LUN (depends on your wishes and design) and use storage vMotion to get the VMs to the new datastore.  Please keep in mind that using Thin provisioned LUNs without using VAAI could cause out-of-space conditions (which can lead to failing VMs) and space on your storage system may being wasted (because space is not reclaimed for example after a migration).
If I have a Thick LUN that is 1.8 TB in space, but is only utilizing 1.4 TB of data; and I want to migrate Thin, do I have to create a 1.8TB thin (oversubscribed) or can I get away with making a 1.4TB thin lun and going a Thick -> Thin LUN storage vMotion?  Thanks, JH
Data size for Storage vMotion (Thick to Thin array side)