Man, sounds like a lot of people are upgrading!
I just did mine a couple months ago: went from a 55 gallon to a 110 gallon reef ready. I had to put the new tank in the exact same spot as the old one, so there were a few logistical problems to overcome. But I managed to do it without starting a new cycle:
Instead of buying a lot more live rock, I went with just a little "new" live rock and the rest dry tufa rock. The base rock would seed into live rock later down the road, but this way I was able to build pre-hung rock towers that were almost completely ready to go in the tank when the final transfer was done.
When the big weekend happened, I drained the 55 into a couple rubbermaid containers, put the HOB filter on one of the tubs and the fish into that tub. The live rock went into the second tub with a couple cheapie powerheads. Then the 55, with the live sand still in it, was moved a few feet away and the 110 put in its place. The Tufa/live rock stands were put in place in the new tank. The sand was transferred to the new tank and mixed evenly with new sand to make up the difference. By the way, I did not have a deep sand bed, or this would have otherwise caused a spike! Sand bed in the 55 was an inch, new sand bed is 1.5 inches to 2.
Once this was done, the tank was filled with a combination of new saltwater and seasoned saltwater form the old tank. I let the system run for 2 days, watching the chemistry, before introducing the fish. Never saw a single spike in amm, trite, or trate when I did this. The tufa rock has since seeded with bacteria and algae, and is well on its way to getting a good coating of corraline. When I look at the tank at night with a red light (and who doesnt?) I see plenty of amphipods crawling around on the tufa. Not as much as on the LR, but I can tell that colonization is in progress.
I hope this gives you an idea of what's in store for you. It's not hard, but it's not exactly a quick project, either. By the way, I have one tip that I wish I had done: If you have access to a vaccum chamber (not common, I know, but any vaccum device that you can jury-rig will help) you might consider pre-soaking the dry tufa rock. Tufa is EXTREMELY porous, and the air that will be trapped deep in the rock is going to take forever to be absorbed and removed. I would recommend putting the tufa in a container with some used salt water, then remove the air from the vacuum chamber to draw out at least some of the trapped gas deep in the rock...this will in effect "suck" the water into the rock. I know not a lot of people have access to vacuum chambers, but you might find something that works. My other hobby is modelmaking, and I use one for moldmaking and casting.
