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;
}

Maybe you send your new companion back to the school or maybe better, you look for a real companion with brain.
A docu for return is here By the way, did you know, AI is the shortcut for... no, no, it means Absent Intelligence!

Assuming, your XML stream never has more  then MAXSTRING characters AND if you can live with a "quick-and-dirty" solution then try this one

Class DC.XML.abc Extends (%RegisteredObject, %XML.Adaptor)
{
Property def As %String;
Property ghi As %String;
Property jkl As jkl;
ClassMethod Test(str)
{
    if $isobject(str) { set:str.Rewind() xml=str.Read(str.Size) } else { set xml=str }
    for{set i=$f(xml,"<![CDATA[",$g(i)) q:'i
        set j=$f(xml,"]]>",i) zt:'j "XMLE"
        set $e(xml,j-3,j-1)="", $e(xml,i-9,i-1)="", i=i-9
    }
    
    set rdr=##class(%XML.Reader).%New()
    if 'rdr.OpenString(xml) write "OpenErr",! quit
    do rdr.Correlate("abc","DC.XML.abc")
    while rdr.Next(.abc,.st) { zzdo abc }
}
}

Class DC.XML.jkl Extends (%RegisteredObject, %XML.Adaptor)
{
Property mno As mno;
}

Class DC.XML.mno Extends (%RegisteredObject, %XML.Adaptor)
{
Property pqr As %String;
Property stu As %String;
}

And some tests...

set s1="<abc><def>010203</def><ghi>040506</ghi><jkl><mno><pqr>070809</pqr><stu>101112</stu></mno></jkl></abc>"
set s2="<abc><def>010203</def><ghi>040506</ghi><jkl><![CDATA[<mno><pqr>070809</pqr><stu>101112</stu></mno>]]></jkl></abc>"
do ##class(DC.XML.abc).Test(s1)
def................................: 010203
ghi................................: 040506
jkl.mno.pqr........................: 070809
jkl.mno.stu........................: 101112
do ##class(DC.XML.abc).Test(s2)
def................................: 010203
ghi................................: 040506
jkl.mno.pqr........................: 070809
jkl.mno.stu........................: 101112

Note: the above ZZDO command takes an oref as argument and prints it, you can replace it with a simple zw oref.