Migrating from VMFS3 to VMFS5, converting thin to thick disks

Hello:

We are in the process of migrating VM disks from VMFS3 datastores to VMFS5.  While migrating, we are attempting to also convert the disks from Thin, to Thick Lazy Zeroed.  However, for some reason the migration will take place just fine but the disks are not being converted to Thick Lazy Zeroed, instead the disks are staying Thin.  Is there a reason for this behavior?  If I then migrate the disks again from a VMFS5 datastore to another VMFS5 datastore, it will convert the disk to Thick Lazy Zeroed.  We were looking to take care of both steps at the same time (a migration from VMFS3 to VMFS5 along with converting the disk from Thin to Thick Lazy Zeroed).  We are on vSphere 5.0.

Thanks for your help.

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Chris