Well, lots of people talk about "cycling the tank", but the tank itself doesn't really do much besides hold water. If you have 30 pounds of live rock, you should be able to move it and have a filter going right away. I would try to keep the live rock underwater at all times, and move quickly so that nothing gets too cold.
Adding base rock to the tank after putting your live rock might be a nuisance, but should be doable.
I guess I'd be sure to have plenty of warm saltwater ready to go, and match the parameters closely to the 29g. You could add it fairly slowly to the 75g to reduce the shock.
If that's scary, you could just move the 29g, reassemble it, and get the new base rock, etc, going in the 75g, although you'll need a heater and some powerheads to keep that tank going.