Data Repoint NULLS Data In New Class Table When Old Class Property Is Changed & Saved

• With %OpenId you create a complete internal local copy of your object
• With %Save it is written back as complete as is was. NOT just changed properties as you might expect And non- defined properties vanish
• Any change that happened in between from some other process is lost
• You may need some locking logic to prevent simultaneous access by 2 instances  eg. locking the Id for exclusive access by 1 single  process.
• In your case it may even happen in 1 process with Old_Class vs, New_Class

Re-reading your question, I understand that properties in 
New_class are not identical and synchronized with Old_class ?
So even Storage Definition might be different ? 

IF YES, I'd say it's highly risky! 

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;
}