Hi I checked with Aegean ,they said the invoice was just to show a change in flight times and it is a standard letter including the new statement about baggage charges.If you booked your flight on the premise of 15kg free for early booking that stands and will appear on your final itinerary/e ticket which you receive 7-10 days before departure.I would keep both original and new invoice as a precaution ,check your itinerary and double check your flight times.Hope this helps.