With ESX and vCenter Server is possible use the "migrate" function to move vmdk files and (if needed) change the type (between thin and thick). But all disks will have the same format. With SVMotion license this operation could also be done the a "live" VM. Without vCenter Server, is possible convert a thin disk to a thick with datatostore browser and the inflate option on the vmdk file (but the VM must be powered off). To convert a thick disk to a EagerZeroed (for FT usage), simple enable FT on a powered-off VM: the disk will be automatically converted!
Note that by default ESX/ESXi disks are ZeroedThick, from vSphere is possible (also from the GUI) build thin disks (choosing "thin provisioning") or EagerZeroed Thick disks (choosing "support clustering feature").