Replace {orgvdc-id} with the ID of the Org VDC you wish to modify.
Locate the current allocation model name within this output.
Copy the body output from above step and replace the old allocation model name with the new desired allocation model name and paste it in PUT call to update the Org VDC with the new allocation model:
PUT https://<vcd_fqdn>/api/admin/vdc/{orgvdc-id}
Execute the GET call once again on the same Org VDC to verify that the allocation model has been successfully changed.