Configure Port-Speed (config_mode) via REST API

[Frank6]

J9855A with YA.16.10.0009:
I am trying to modify a switchport's speed setting (10/100/1000/10G/auto) using the ArubaOS REST API
PUT /ports/{id}
However, when providing the corresponding "config_mode: ...." in the request body, the API answers with
400 Bad Request
{"message":"Read only attributes cannot be changed"}
According to the API docs, "config_mode" is writable. Setting it via CLI also works as expected.
Am I missing something?