When i have changed substrates i tend to do a complete overhaul of everything.
This is what i would do and the way i would do it.
Turn off all heaters and filters and place into a bucket of old tank water (ensure the heater has cooled first)
Remove as much water as you can without affecting the fish's ability to stay upright
Remove any decor and plants (plants into a bucket of old tank water)
Remove fish into a bucket of old tank water (will be fine for 4 hours without heater or filter)
Remove the rest of the water
Remove old substrate
Clean thoroughly
Add pre rinsed substrate
Add any hard decor like rocks and wood
Start to fill by pouring dechlorinated water (or siphoning) onto an upturned dish/plate
Add plants etc
Add heater and filter back to the tank
Ensure water temperature is correct
Re-acclimatise fish back to their 'new' tank
A couple of days later clean your filter and filters' media to remove any particles of sand which would damage the motor and impeller. Remember to do this in old tank water though.
Also keep an eye out for any new battles happening if you have aggressive fish as they will probably have to sort out who is boss again and what their territories are.
Have fun!
