Are you going to remove the local storage on which you currently have ESXi installed?
From what I understood the VM's are on the same local storage as the Hypervisor? The only advantage I see in installing the Hypervisor on USB/SD is to run the host diskless, in case you have shared storage. If you are going to install ESXi on USB/SD with the current installation still on the disk, you might receive an error message during installation about a second ESXi instance being present.
André