Change reservation has been added to Cloud Client 3.3. Which might be helpful over here.
For more information, see the release notes: https://www.vmware.com/support/vcac/doc/vrealize-cloudclient-33-release-notes.html
Command to Change reservation of the VM and storage path:
vra provisioneditem change reservation --ids 340d7075-728b-4d39-96c6-3a5abee7f360 --blueprintName testb --reservationName "VC55-Cluster-Res-1" --storagePath "path1"
Thanks
Kumaran