https://hcpanywhere.hitachivantara.com/a/WRFYnbyRURRLEVic/Subnets%20ethX%20%26%20vlan%20combinations.docx?l
You cannot change the VLAN-tag of an interface, you first have to delete that interface. But you cannot delete the mng0 in order to re-add it along with the VLAN tag. The Installation manual only shows examples of VLANs configured on data ports.
Instead, your customer could use an ethX port to manage the HDI
Kind regards, Henk