Write to Utf8JsonWriter from Utf8JsonReader

Manipulate.cs

var reader = new Utf8JsonReader(Encoding.UTF8.GetBytes(json));

using var stream = new MemoryStream();
using (var writer = new Utf8JsonWriter(stream))
{
    while (reader.Read())
    {
        switch (reader.TokenType)
        {
            case JsonTokenType.None:
                break;
            case JsonTokenType.StartObject:
                writer.WriteStartObject();
                break;
            case JsonTokenType.EndObject:
                writer.WriteEndObject();
                break;
            case JsonTokenType.StartArray:
                writer.WriteStartArray();
                break;
            case JsonTokenType.EndArray:
                writer.WriteEndArray();
                break;
            case JsonTokenType.PropertyName:
                if (reader.GetString() is string key)
                {
                    writer.WritePropertyName(key);
                }
                else
                {
                    throw new InvalidOperationException("Property name is not a string. This is most likely a programming error.");
                }
                break;
            case JsonTokenType.Comment:
                writer.WriteCommentValue(reader.GetComment());
                break;
            case JsonTokenType.String:
                // If it's English-only:
                writer.WriteStringValue(reader.ValueSpan);
                
                // Works for åäö at least:
                writer.WriteStringValue(Encoding.UTF8.GetBytes(reader.GetString() ?? string.Empty));
                break;
            case JsonTokenType.Number:
            case JsonTokenType.True:
            case JsonTokenType.False:
            case JsonTokenType.Null:
                writer.WriteRawValue(reader.ValueSpan);
                break;
            default:
                throw new NotImplementedException($"Havn't implemented {reader.TokenType} yet.");
        }
    }
}

var copiedJson = Encoding.UTF8.GetString(stream.ToArray());

writer.WriteStringValue(Encoding.UTF8.GetBytes(reader.GetString() ?? string.Empty)); makes double escape on , so \n becomes \n and so on.