LiveCycle will extract all bound data fields into the XML data. I don't believe that you can change data binding at run time. One thing you could do is to have a process that evaluates the data coming from the form by examining one of the fields. If the data matches a 'file A' type then just pass along the data. If it matches a 'file B' type then run an XSLT transformation to get the other form of the data.