A properly mineralised substrate may last a year or two, you'd be lucky to get 6 months out of the average dirt.
Just like ordinary fertiliser, root tabs take a while to dissolve and start working., so don't expect an immediate turnaround.
Good luck with the redo, it doesn't have to be a trainsmash, though you certainly have your job cut out for you.
I'd start by planning where I'd want to move the existing plants, any hardscape moves etc.
Get your new substrate ready. Save some water to buckets/tubs/bathtub and move livestock there.... drain water till you hit the substrate, it would be unreasonable to expect to be able to keep half of the water considering the volume of the tank... but a bathtub could take almost 50 gal, you just need a pump and some pipes to transport the eater through the house.
Once you hit the substrate, dig a hole till you reach the glass... stick wet/dry vacuum cleaner nozzle in the hole and suck out the last water from the substrate, it will gradually let it drain into the hole. Pull and store plants. Go check on livestock. Now get the old dirt out and new dirt in. Replace hardscape and replant. Fill, heat and bring back the livestock... try to keep the glass moist, but that is probably not realistic due to the time the work will take just emptying and filling such a large tank, so you better keep the filter running in some old tank water to keep it "alive". I'd say it could be done in the coarse of a morning/maybe half a day.
And it is fun after all. Clean slates are very liberating.
Cape Town, South Africa.
Hi. I'm back.