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