From the update of TRTH support team, you can use TickHistoryTimeAndSalesExtractionRequest to get the Reference & Symbology Change data. The content fields are:

        {
            "Code": "THT.Reference Change - Change Type",
            "Name": "Reference Change - Change Type",
            "Description": "Reference field changed",
            "FormatType": "Text",
            "FieldGroup": "Reference & Symbology Change"
        },
        {
            "Code": "THT.Reference Change - New Value",
            "Name": "Reference Change - New Value",
            "Description": "New value as a result of the Reference change",
            "FormatType": "Text",
            "FieldGroup": "Reference & Symbology Change"
        },
        {
            "Code": "THT.Reference Change - Old Value",
            "Name": "Reference Change - Old Value",
            "Description": "Previous value in the Reference change",
            "FormatType": "Text",
            "FieldGroup": "Reference & Symbology Change"
        },

The request looks like:

{
  "ExtractionRequest": {
    "@odata.type": "#ThomsonReuters.Dss.Api.Extractions.ExtractionRequests.TickHistoryTimeAndSalesExtractionRequest",
    "Condition": {
      "ApplyCorrectionsAndCancellations": false,
      "DisplaySourceRIC": true,
      "MessageTimeStampIn": "GmtUtc",
      "QueryEndDate": "2017-08-17T23:59:59.999Z",
      "QueryStartDate": "2014-08-17T00:00:00.000Z",
      "ReportDateRangeType": "Range"
    },
    "ContentFieldNames": [
      "Reference Change - Change Type",
      "Reference Change - New Value",
      "Reference Change - Old Value"
    ],
    "IdentifierList": {
      "@odata.type": "#ThomsonReuters.Dss.Api.Extractions.ExtractionRequests.InstrumentIdentifierList",
      "InstrumentIdentifiers": [
        {
          "Identifier": "KLOV.ST",
          "IdentifierType": "Ric"
        }
      ],
      "UseUserPreferencesForValidationOptions": false,
      "ValidationOptions": {
        "AllowHistoricalInstruments": true
      }
    }
  }
}

Thomas - did you ever figure this out? We are trying to do the same.