AIS formservice request to P4210_W4210A Add Mode vs Update Mode

I am trying to use an AIS formservice request to update an existing sales order. I have formServiceAction = U but the form is still throwing error 0002 because the form appears to be in add mode instead of update mode. Is there a way to call the form in update mode to edit quantity/price on detail lines?

POST to <ais_endpoint>/jderest/v2/formservice

{

    "environment":"JDV920",

    "role":"*ALL",

    "username":"USER",

    "password":"****",

    "formName" : "P4210_W4210A",

    "version" : "ATK0001",

    "formServiceAction" : "U",

    "bypassFormServiceEREvent" : "true",

    "formInputs": [

        {

            "value": "3219",

            "id": "1"

        },

        {

            "value": "SO",

            "id": "2"

        },

        {

            "value": "00020",

            "id": "3"

        },

        {

            "value": "         211",

            "id": "5"

        },   

        {

            "value": "USD",

            "id": "17"

        }       

    ],

    "formActions" : [

            {

                "gridAction":

                    {

                        "gridID": "1",

                        "gridRowInsertEvents": [

                            {                   

                                "gridColumnEvents": [

                                    {

                                        "value": "10",