Assuming you mean to conver the disk to Thin as in thin provisioning, rather than actually change the size under Edit settings for the VM, you can do this with svMotion.
If you actually want to shrink the disk, so the guest sees a smaller disk... Not sure.
If you are in Windows, I believe you can defrag (probably need a 3rd-party tool to move everything to the front of the partition), then use diskpart (or Disk Mgmt if you're on 2008) to shrink the partition. Then, you might be able to use VMware tools to shrink the VMDK file.
Of course, I haven't tried it myself yet. It's a fairly common technique, though most often applied when the OS is offline by using a LiveCD or mounting the HDD/VMDK to another system/VM.
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