Serialization & backward compatibility (DataContractJsonSerializer)

Question

I am using DataContractJsonSerializer.

Currently, my serialized object contained a member of class "Settings"

Now, I would like to extend my support and serialize any class that implements ISettings interface.

[DataContract(Namespace = "")]
[KnownType(typeof(SystemSettings))]
[KnownType(typeof(PrivateSettings))]
public class Data
{
    [DataMember]
    public ISettings settings { get; set; }
}

public interface ISettings
{
}

[DataContract(Namespace = "")]
public class SystemSettings : ISettings
{
    [DataMember]
    public string Name { get; set; }
    [DataMember]
    public string SysName { get; set; }
}
[DataContract(Namespace = "")]
public class PrivateSettings : ISettings
{
    [DataMember]
    public string Name { get; set; }
    [DataMember]
    public string PrivateName { get; set; }
}

This works perfectly fine, as the serialized data contains the type of the object

<?xml version="1.0" encoding="utf-8"?>
<root type="object">
     <settings __type="SystemSettings" type="object">
         <Name>System Settings</Name>
         <SysName>New System</SysName>
     </settings>
</root>

My issue is with backward compatibility. The existing serialized files do not contain the object type (__type="SystemSettings"). When deserializing old data into my class, I get

"{"Unable to cast object of type 'System.Object' to type 'JsonSerializer.ISettings'."}"

Is there a way to solve this? Can I direct the serializer what default type to instantiate for the interface?

Thank you!