I changed a 40 long aquarium that I'd inherited care of for a 50 Long last winter. The 40 turned out not to be an aquarium, but a reptile tank and the plastic bottom was sagging ominously. Here's the proceedure I used:
1. Mix 25 gal of clean saltwater the night before.
2. Fill my 10 gal with tank water, and start filter for circulation. Corals were tranfered to the 10 gal.
3. Fill rubbermaid tote with water and move live rock to it.
4. Remove fish from now half empty tank with nothing but gravel subtrate (made them easy to catch). Transfer to 10 gal tank with corals.
5. Finish draining old tank and remove with old crushed coral substrate.
6. Set new tank on stand, move live rock to new tank, and partially fill with water that the live rock was in.
7. Add new clean sand for shallow sand bed and finish filling with clean salt water. Let filters run for a couple hours to clean cloudyness from sand.
8. Aclimate corals and fish and return to tank.
Took about 6 hours total, didn't loose a thing and no ammonia or nitrate spikes afterwards.
Phil