How do I exclude unchanged fields in Flashback query resultset?
Is there a way to excluded unchanged fields from versioned response?

Not really.
Flashback uses the information found in UNDO. Flashback Data Archive stores the results from the Log files in a table.
So, you get ALL of the data when you do a Flashback query.
The common method is to use LAG + DECODE.  But, I'm afraid that this will problematic for your LOB columns.
Personally, I'd pawn off the problem to the UI (Display Tier).