This article provides the procedure to move a vSAN Witness Appliance to a different vCenter.
VMware vSAN 7.x
VMware vSAN 8.x
vSAN Witness is a virtual appliance, and it does not hold any user data but just metadata. It is advisable to deploy a new witness appliance on the destination vCenter and replace it by using the steps in the KB below:
vSAN Stretch Clusters—How to Replace the Witness Appliance/Host