Welcome to Flyertalk indeed and hope that you'll find it a useful place! If you book a trip today for travel in a few months and become gold again, would you not expect BA to then give you access to all Gold seats on that flight again? The situation is exactly symmetric: all advantages (including lounge access, seat selection, luggage allowance, etc) will be dictated by your status at the time you fly, not your status at the time you booked.