In the past DVLA have only required supporting evidence on a change of cc when it has affected the taxation rate of the vehicle by going to a lower class, if it went to a higher class it was just accepted. It has been their policy to request written confirmation from the garage that carried out the work and to withhold the issue of a new V5C until their request has been met.

They have now changed that policy to include INCREASES in engine cc even for Historical Class registered vehicles where, currently, it doesn’t affect taxation or usage.
Also you now need to provide the same proof of change of fuel type.

Current details available from :-
DVLA - Changes to registration certificate