I think that the biggest obstacle is time.
Time to let the new water to cycle.
Time to let the glue for the plumbing fittings cure.(as long as you already have the plumbing layout and all the fittings right there with you and don't have to make a million trips to the fitting store
)
When I moved my 204g last year, I also had that empty 180g, which I moved first. I did have time on my side, but I can't imagine moving without having a spare tank, and a 1 month overlap. You could transport the old water, but like UnderWorld says, what a hassle. Your precious fish get stressed, your corals react, you have heat issues, water movement and quality issues....
oh yeah...then you need to move your furniture too.
Mitch