using System;

public class Program

{

    public static void Main()

    {

        var o = new 

        {

                Date = DateTime.Parse("2019-08-01"),

                TemperatureCelsius = 25,

                Summary = "Hot"

        };

        string sysString = System.Text.Json.JsonSerializer.Serialize(o);

        string newString = Newtonsoft.Json.JsonConvert.SerializeObject(o);

        Console.WriteLine(sysString);

        Console.WriteLine(newString);

        var j = Newtonsoft.Json.Linq.JObject.Parse(sysString);

        Console.WriteLine(j);

        newString = System.Text.Json.JsonSerializer.Serialize(j);

        Console.WriteLine(newString);

        var opts = new System.Text.Json.JsonSerializerOptions 

        { 

            Encoder = System.Text.Encodings.Web.JavaScriptEncoder.UnsafeRelaxedJsonEscaping,

            Converters = 

            {

                new JTokenJsonConverter()

            }           

        };

        newString = System.Text.Json.JsonSerializer.Serialize(j, opts);

        Console.WriteLine(newString);

        j = System.Text.Json.JsonSerializer.Deserialize<Newtonsoft.Json.Linq.JObject>(newString, opts);

        Console.WriteLine(newString);

    }

    public class JTokenJsonConverter : System.Text.Json.Serialization.JsonConverter<Newtonsoft.Json.Linq.JToken>

    {

        public override bool CanConvert(Type typeToConvert) => typeof(Newtonsoft.Json.Linq.JToken).IsAssignableFrom(typeToConvert);

        public override Newtonsoft.Json.Linq.JToken Read(

            ref System.Text.Json.Utf8JsonReader reader,

            Type typeToConvert,

            System.Text.Json.JsonSerializerOptions options) {

            using System.Text.Json.JsonDocument document = System.Text.Json.JsonDocument.ParseValue(ref reader);

            return Newtonsoft.Json.Linq.JToken.Parse(document.RootElement.GetRawText());

        }

​

        public override void Write(

            System.Text.Json.Utf8JsonWriter writer,

            Newtonsoft.Json.Linq.JToken obj,

            System.Text.Json.JsonSerializerOptions options) => 

                writer.WriteRawValue(obj.ToString(options.WriteIndented ? Newtonsoft.Json.Formatting.Indented: Newtonsoft.Json.Formatting.None));

    }   

}