Symptoms:

When changing volume provisioning type from thin to thick and from thick to thin, the changed volume goes into an initial sync

Cause:

This flow is currently by design, but planned to be changed in the future.
The same flow applies to changing provisioning on the VC/vCD managers or on the Zerto "Edit Volumes" dialog replicating from any platform 

Solution:

Currently no workarounds available.
Zerto is planning to implement a GUI alert that notifies the user about a volume initial sync result by this action in one of the future releases

Affected Versions:

All versions

Hypervisor:

VMWare/Hyper-V