Use everything from inside your existing tank, sand, rock, BIO MEDIA!!!! All that stuff has the beneficial Nitrifying Bacteria on it we need in the system to eliminate the waste toxins. Not to mention all the time we spend GROWING the Algae before we ever add anything to the tanks. You want ALL that entire year's algae/bacteria growth into the new tank. Doing this eliminates the need to Cycle the tank. When you cycle, your growing algae/bacteria on the rocks and sand right? Well since you already have completely cycles rock and sand, all that needs to happen in the new tank is for the algae to grow into the new sand and rock you will be adding because you have more space, and more water. I like to keep close to the 1lb sand +1lb rock per gallon of water ratio.
The only thing that you gain no benefit from transferring from one tank to another is the water itself. Unless you have incredible Pod density in your water column.
I'm upgrading from a 75g to a 300g myself. started the tear down of the big tank yesterday, draining it today and 40 or so Mbuna Cichlids getting re-homed. I had a wall in front of the big one. removed it yesterday. I'm documenting this in a Build Thread if that matters to anyone but me
Anyways good luck to you on the upgrade