I think there is an easy solution to this. Cure the rock in another container. I used a big plastic kegerator with a heater and powerhead when I upgraded from my 36 to my 55. This gave me the side time to scrub the rock down well between shifts at work and when I was happy with what was going on, toss everything into the new tank.
Similar concept with when you have to take down the smaller tank, 5 gallon buckets for the existing stock and water from the tank. Make the move quick enough you won't need the heater.
Only thing that will be a pain is sand. I never suggest using old sand...just nitrate traps. To keep things stable you'd want the rock in first and THEN pour sand in... This will be a mess, of course, but using some PVC
piping to gently pour the sand into the bottom will help prevent any sand storms.