Updated Aug 17, 2022: VMware HCX User Manual & Documentation has been amended to reflect updated disk UUID behavior.

4.2 RN Update  Disk UUID Retention
     For Bulk and RAV migrations, disk UUIDs are carried forward with the virtual machine.
4.2 – 4.4 User Guide Updates
Disk UUID restrictions removed from the User Guide

Updated June 18, 2022: With HCX 4.2 – all HCX migration types will retain original disk UUIDs. This was a capability that was missed (not announced) when VMware HCX 4.2 was released.

What are you talking about Gabe? WHAT ARE VM UUIDS?

Universally Unique Identifiers are generated for each virtual machine disk when a VM is created. You can see the identifiers by running the vmkfstools on the on the ESXi host:

What happens to the virtual disk UUIDs during HCX Migrations?

• HCX vMotion & Cold Migration
  With HCX vMotion, the disk UUIDs are not changed. The HCX Mobility Agent uses the source site vm config spec; there is a vim.vm.device.VirtualDisk configuration that references the original disk UUID.
  
• HCX Bulk Migration and Replication Assisted vMotion (RAV)
  
  With HCX Bulk Migration, the disk UUIDs will change are not changed.
  As of HCX 4.2, the replication engine used in Bulk and RAV migrations has been enhanced to set the disk UUIDs on newly created based on the original VM disk UUID. The default/previous behavior of the engine is to generate new disk UUIDs.
  
  With RAV migrations, the disk UUIDs will change. During the migration switchover, when HCX vMotion is engaged for delta vMotion, the replicated virtual machine will have already been created with new disk UUIDs using the same approach as with HCX Bulk migration.

Why does this matter?

There are multi-disk virtual machines with OS level storage management elements out there. Those may contain disk UUID references. Changes to the disk UUIDs in this case can be disruptive to those storage management functions.

It is recommended to use the HCX vMotion when the applications have disk UUID related dependencies.