Vinegar? Peroxide? i know diluted they are ok......not sure if a very small amount of bleach is ok or not....
I would try a little bit of vinegar and water give the tank a good scrubbing let it dry really well....and if you are changing to a different substrate you don't really have to worry about the old stuff.... right here and now anyway.
if you are still concerned turning the heat up on the tank and maybe putting some parasite meds or aquarium salts in to kill off any potentials in there still.
I'm mostly brainstorming and trying to remember what i have read about situations like this.....i'm sure some much smarter than me will chime in