Keeping existing entries when serializing in a json config file with Newtonsoft.JSON

I am struggling a bit whith the handling of a JSON settings file. Currently we are deserializing a settings file on program startup into a defined object and serializing the object to the file when the application terminates. From time to time we are changing the object structure (mostly adding new entities but sometimes removing obsolete entries, etc.) and this causes confusion to our developers as well as power users which are working with different releases (or development versions) because the settings file is always overwritten and forced to the current application versions' format.

Now my question: Is there a good way to keep json entries that the currently running application version does not "know"? I already thought of achieving this behavior by only updating those entries that the current version knows and not touching others that are already in the file. Or by keeping a list of objects in memory the parser does not know after deserializing (this could be a little bit more complicated because of the different serialization of different data types) and the serializing those entries. But I am not a Newtonsoft expert so I do not know how to get this into code.

Does anyboady have advice or a kind of best-practice for this problem?

A solution is to use the attribute JsonExtensionData. This attribute indicate to set unmapped property here.

string json = @"{
  'UnknowProp1': 'Some value',
  'Id': 1,
  'Label': 'Hello',
  'UnknowProp2': 'Other value',
}";

var dto = JsonConvert.DeserializeObject<Dto>(json);
var result = JsonConvert.SerializeObject(dto, Formatting.Indented);

Console.WriteLine(result);


public class Dto
{
    public int Id { get; set; }
    public string Label { get; set; }

    [JsonExtensionData]
    private IDictionary<string, JToken> _additionalData;
}

Result :

{
  "Id": 1,
  "Label": "Hello",
  "UnknowProp1": "Some value",
  "UnknowProp2": "Other value"
}

But the order isn't respected.