Select XML from different columns and return into one XML

I have a Table "Action" which has multiple columns. Some of columns are:

Id            (Number)
Name          (Varchar2)   
Old_Action    (XMLTYPE)   
New_Action    (XMLTYPE)  {New Column added}
Action_Merged (Varchar2) {New Column added}

There is an existing oracle procedure that returns Clob data successfully as follows:

CREATE OR REPLACE PROCEDURE GET_ACTION (ID IN NUMBER, OUTPUT OUT CLOB)
AS
XMLOUTPUT XMLTYPE;
v_id Action.Id%TYPE;
v_query CLOB;
BEGIN
 v_id :=ID; 

 v_query := 'SELECT XMLElement(
     "CustomerFile",
       (SELECT XMLAgg(
                 XMLElement(
                   "Customer
       act.OLD_ACTION'.extract(''//CustomerType/*'',''xmlns="http://ift.tt/1qhYN2q"'')
                   ))
          FROM Action act
          WHERE act.ID = '  || v_id || ' ))) FROM DUAL';              
EXECUTE IMMEDIATE TO_CHAR(v_query) INTO XMLOUTPUT;
OUTPUT := XMLOUTPUT.getClobVal();
END GET_ACTION;

There is an enhancement request such that two new columns are added New_Action and Action_Merged. Initially only column Old_Action contains xmldata. But now there may be scenarios where New_Action columns will also contains xml data. Following are the conditions for data in both columns Old_Action and New_Action :

Old_Action       New_Action       Action_Merged
<old xml data>   <null>           <null>
<old xml data>   <new xml data>   <MergedIds>

Note:<old xml data> and <new xml data> are the xml containing Customer specific data with different xml nodes. <old xml data> and <new xml data> are just short form. 

When New Action is merged, MergedIds are inserted into Action_Merged along with new XML data in New_Action column.

Problem is that If New_Action column contains data, then GET_ACTION procedure should return data from New_Action otherwise from Old_Action column in a single XML output. And there may be multiple records same Ids with Old_Action or New_Action

Sample data from Action table:

Id     Name    Old_Action       New_Action      Action_Merged 
1      A       <old xml data>
1      B       <old xml data1>   <new xml data>  <1,2,3>
1      C       <old xml data2>   <new xml data1> <4,5,6>
2      D       <old data>

and so on. Expected result from GET_ACTION procedure:

<CustomerFile>
    <old xml data>
    <new xml data>
    <new xml data1>
</CustomerFile>

It means if Procedure is called for Id "1", then must return resulting xml containing old_xml_data for name A, new_xml_data for name B, and new_xml_data1 for name C.

Please help.

Thanks in advance.