Upgrade isn't hard at all...the established rock and such means you don't have to wait for a cycle. The amount of good bacteria already matches what is in your tank. When I moved from Wisconsin to Illinois, I went from a 75g to a 90g...all moved in one day. Put wet saltwater paper towels over the rock, and maybe a half inch of water in the bottom of the rock tub, you shouldn't have any die off to worry about. The tiny critters can live a little out of the water and find a hole or cranny in the rock to keep themselves alive...the little amount of water in the tub will give them an even better chance if they jump off the rock.
Have new mixed saltwater for the tank all set before you move, waiting at the new place. It's a little bit of a hassle, but I made a trip the night before the move to the new place do that. With the water mixed through the night and waiting, all I had to do was break down the tank and move in...first thing, reset the tank or the new one, and get the critters back into their new home ASAP.
The secret to an easy move is to not clean anything, don't change the media, change out nothing. The sand, if are worried about toxins, swish it around in the old water before removing it. That will release anything in the sand. You can even add some more new mixed clean saltwater, and swish it around again if you really want to clean it up. It's very tempting to want to clean everything up once it's all broke down...don't do it.
You don't need to keep any old saltwater, there is nothing wrong with replacing 100% of the water. The good bacteria is on the rocks, and sand, as well as all the media and plastic (power heads and such). The good bacteria on all of those things will prevent a spike.
As Jay suggested...keep some ready mixed saltwater on hand, just in case you do get any kind of spike and need to do a water change...I never did, and I moved 5Xs already, but I was prepared just in case. In all my moves, the only thing I ever lost was a cleaner shrimp, one was fine, but the other just fell over dead just before I was ready to transfer them into the new digs. Fish go in separate bags, each coral in it's own bag (stressed corals release toxins for chemical warfare, so even little frags need their own bag) and rock in a tub. Acclimate the fish and coral to the reset tank, just like you did when you fist got them.
