My own opinion (may not be best) take the fish out and continue to run your filters in whatever container you have the fish in. Get all old substrate out. Put in sand (disclosure: I don't think there will be any kind of chemical agents or whatnot that would come off of the sand but if so then could be hazardous adding fish soon after adding sand... I'm a neanderthal though... in my case I would add fish right after sand and if they died then I would use it as an excuse to get brand new discus against my wife's wishes).
Now with old substrate gone and sand put in, add "safe" powder (twice recommended amount) and start to fill that bad boy up. Personally, I would use about 5 or 10 gallons of old water. Probably 5 gallons since fish are stressed with what's going on and probably ammonia coming out of them is a little higher than normal. Next, go ahead and transfer your filters back to the aquarium. Whatever Driftwood or plants make sure to put them in before aquarium gets filled all the way. Then throw the fish back in. If they make it, cool! If they don't, buy new ones from great sponsors on here!