I don't know anything about your tank, how big, what is in it, etc.? But, I would do try to do it in one shot. I would use the same water (but if you have time/space and want to, then use all new). Make sure everything is in place and ready for the change over. I am assuming you are using the same area, just moving the old one and putting a new/bigger one in its place?
Have your extra water mixed up before you change over, ready to use/go. I would transfer everything that I planned on changing over, water, rock, corals and fish to 15 to 20 gallon rubbermaid type containers. These are small enough to move around a bit to get them out of the way after you transfer things to them. Move them with lids on if possible, makes it a little stronger and keeps them from bowing or water, etc form sloshing out. Move the old tank/stand out of the way and move the new one in.
If you are planning on adding any new sand or rocks, I would add it first/now. As well as the extra water. Then I would add the old water, rocks, fish, etc. Not much to it, except a little labor intensive. I would not worry too much about a cycle. I am sure you will get a very small one, but should not be big enough to be concerned about if you keep all of your stuff submerged in the water during the move over.
I did this exact thing (well, almost) 3 or 4 (maybe 5) months ago. I did not move my tank or upgrade, but I did change out all (and I do mean all) of my sand and rockwork and replaced it with completely new dry sand and rocks. I did it this exact way that I describe to you. My only difference is that I did not keep any rockwork, etc for cycle. My tank went through a complete cycle, which yours will not, if you do not replace your rocks, etc. I lost no fish (3) or corals (only had a couple of mushrooms and polyps, though) to this changeover.
Good luck, Frank.
