How to update only a few fields with JSON import?

[iotim]

I'm getting the following error:

"1 of 1 issues have been skipped because they already exist in destination projects."

All I'm sending is the key and a custom field.  Any advice?  Is this possible?  Below is my sample JSON.

{
    "projects": [
        {
            "name": "MyProject",
            "key": "AB",
            "issues": [
                {
                    "key": "AB-50",
                    "customFieldValues": [
                        {
                            "fieldName": "User\/Customer",
                            "fieldType": "com.atlassian.jira.plugin.system.customfieldtypes:textfield",
                            "value": "Test"
                        }
                    ]
                }
            ]
        }
    ]
}

[iotim · Accepted solution]

Well, I don't think you can.  Instead I wrote a php script using curl and basic authentication.  For the next guy, the following links helped me.  

https://developer.atlassian.com/cloud/jira/platform/basic-auth-for-rest-apis/

https://developer.atlassian.com/cloud/jira/platform/rest/v3/api-group-issues/#api-rest-api-3-issue-issueidorkey-put

[iotim]

Can anyone help with this?   I really don't want to write an app just to do this simple task.  I looked into CSV import to alter a single field, but it says that if you do not send all the fields, the ones not sent are set to their defaults.  I can't believe there isn't a method to do an update via JSON.