Sorry to disagree here but the DVLA would never ask you to destroy old documents -- they would ask you to return them for replacements yes but never destroy.
they would need the original back in order to send out the new one.
You sure you haven't been the victim of a hoax or scam?
To quote from the letter (INS215) that came from DVLA with my new red V5C:-
Q.What do I do with my old blue V5C?
A.You must destroy the blue V5C for this vehicle now.
The point I was trying to make was that everybody could keep there old blue V5C but they would not give me back the old one for historical purposes.