The C-5 is capable of taking off at the wartime max gross weight of 840,000 pounds with no problem (as long as the necessary field length is available). Actually, "I" feel the Galaxy is capable of taking off at over one million pounds gross weight! But, heavier gross weights put a lot of stress on the wings, therefore shortening their lifespan. The 769,000 pound GW limit is in place to ensure the wings aren't unecessarily overstressed. Also, the 769,000 pound limit ensures that the C-5 can takeoff from just about any "improved" airfield in the world. By "improved" I mean it must be made of concrete or something of the sort, otherwise the aircraft could sink at heavy gross weights.
The heaviest GW that I ever took off at was just under 800,000 pounds during the Gulf War. We had two M1-A1 Abrams tanks, 73 PAX, and a lot of fuel on board.
BTW, there are no "switches" to flip.