From this guideline, one may set the service interface to be serialized with DataContract, and then all rest of referenced classes from the methods's API would inherit such serializer as well, unless overriding specific classes with e.g. [XMLSerializerFormat].

But, is it possible to do the opposite? That is, keeping the main service interface with the XMLSerializer default serializer, and override few classes to use DataContractFormat. Per my sample application (and testing), doing so does not seem to switch to the DataContract (doesn't syncronize the attributes to the client's service reference).

It's for an attribute like this:

//[XmlIgnore]
[DataMember]
public string Var
{
    get => _var;
    set => _var= value;
}