First, there is no NaN in JSON.

set dynObj = {"val":($double("NAN"))}

write dynObj.val + 123 ---> "NAN

zwrite dynObj

dynObj={"val":($double("NAN"))}  ; <DYNAMIC OBJECT>

/* Question @devcommunity manager: how to exit the "insert code block" function? */

Second, red this for more understanding

I'm not sure, do I understand you correctly... 
If you had a class and copied that class to a new class with some more properties, something like this

Class DC.OldData Extends %Persistent
{
Property FirstName As %String;
Property LastName As %String;

/// Redirect the %Open(), %OpenId() to the new class
///
ClassMethod %OnDetermineClass(oid As %ObjectIdentity, ByRef class As %String) As %Status [ ServerOnly = 1 ]
{
	Set class="DC.NewData"
	Quit $$$OK
}

/// Prevent creating a new instance of the old class
Method %OnNew() As %Status [ Private, ServerOnly = 1 ]
{
	Quit '$$$OK
}
}

Class DC.NewData Extends %Persistent
{
Property FirstName As %String;
Property LastName As %String;
Property MartialStatus As %String;
}