I have a 36 gallon bowfront that I recently redid. To start, I had one goldfish several Amano shrimp and a few nerite snails. I decided I wanted plants (yes, goldfish, shrimp, and plants - she is really good with plants and the shrimp). I decided to go with eco-complete and the gravel I had in there.
I started by removing most of the objects so I could catch everyone easier. After placing everything in a 5 gallon bucket with tank water, I removed the fauna to individual buckets (of tank water).
I then started to gravel vac and water change (I took out about 60%). I scooped out the gravel and poured it into the bucket with the eco-complete. I mixed well (including the water in the bag - no rinsing).
Once the substrate was in I placed in new objects and the old ones I was to reuse. I then started putting in clean water and planting at the same time (I had a friend helping). Once the tank was set up with everything except the fauna, I turned on the filters (two HOB aqueon 30) and let it run for about an hour before I returned fauna to tank.
I should note I did had a spare bucket of tank water that I constantly replenished the holding bucket's water with.
Also, I did have a small ammonia spike for a day or two and lost one shrimp (surprisingly the one orange shrimp [caridina propinqua] survived). Everyone is happy and Nuke (the Shubunkin) has eaten a few plants but left the rest alone. The shrimp and snails are alive and well.