I guess it's time to chime in here...having just moved my 240g from seattle to san jose for the second time and having moved a heavily stocked tank 10 times in 9 years I'd say I have a bit of experience with this. That aside, everyone has brought up some very valid points. Moving extra water is definately a pain in the butt and has very limited added value. I still always use ~ 25-50% of the old water. The water in the bags that you move livestock is of no value once you move as most of the animals have polluted it up to the point that it does more harm then good (slime and poop YUCK). New water should match your old parameters EXACTLY (same s.g., temp, alk, Ca, Mg) I always rinse liverock off before I pack it for the move in a bucket or two of old tank water to remove detritus that's built up. I've used the old sand and started fresh. I think starting fresh with some old to seed is the best way. A trick I've learned is if you put a layer of new sand, layer of the old stuff and then a layer of new sand it helps jump start the cycle of the sand. No matter what you do there are always some spikes, bacterial die off, and mini cycles in the new tank but that's how the hobby goes. Understanding water chemistry and proper flow are probably two of the most important aspests of this hobby (even more then lighting!) The most IMPORTANT aspect however is that each tank is unique, everyone's approaches are unique. There are no real right or wrong or failsafe ways to do anything in this hobby but research and asking those who have made the mistakes already definately helps to make things more enjoyable and successful. IME
ps...diverphil71 helped me to make this last move one of my most successful to date!