I completely removed an UGF (45 gallon tank) and all the old gravel - replacing it with 2 canister filters, fine substrate with laterite.
I ran 1 of the canisters on the tank for a few days prior to the changeover.
I put 1/3 of the water into a bucket and added the driftwood and attached plants. I put 1/3 of the water into a bucket and added the fish. The remaining 1/3 of the water I drained.
I completely removed the old gravel and UGF down to the bottom glass (wiping it clean). I added new substrate and laterite. I next filled the tank 1/3 of the way full and added the driftwood and plants (planting the remaining substrate heavy at the same time). I then added the next 1/3 of water and the fish. I then filled the tank up the remainder of the way with fresh treated water. I connected the canister and added the second and never missed a beat.
Although a 9 hour project, my water parameters didn't change and I didn't lose a fish.
The tank now (8 weeks in) looks very, very good. The bacteria, etc. left in the canister and water outweighed that in the substrate that I removed and the changeover went without a hitch.