I'm sure there are many ways to do it, but I would move the 29 gal
to a different location (after removing the live rock and as much of the water as you can without exposing your fish to air) so you can place the 75 gal
where you want it (pain in the butt I know, but you said you wanted the 75 in the spot the 29 was occupying). Put the water you emptied out of the 29 back into the 29. Fill the 75 Gal
tank with new water and put your live rock in it. The live rock will cycle the tank. Since you are increasing to 75, you're going to need another 50 lbs or so of live rock. I have heard that you can mix live rock with dry to save some dough and minimize the risk of unwanted hitchhikers. Once cycled, add the fish. Your also going to need a lot more sand. Avoid the gimmicky "live" sand.