How to clone thick prov disk to thin when not enough space

Hi all.

I have a need to change a thick provisioned disk to a thin.

I get the process of using CONVERT to clone the disk, swap to the new disk in the VM settings then delete the original thick disk HOWEVER even when it is thin the disk will be bigger than the available space I have left.

Is it possible to do this and have the new "cloned" thin disk saved to external storage just until I can then delete the original thich disk and move the thin disk back to the volume??

I am running ESX/vsphere 6 on a IBM Blade S.

thanks

Tracy