Exactly what I will be doing with my tank for my new scape over the next month or so.
Will involve removing plants, removing fish, removing old water and substrate, adding new substrate, adding new water to ½ tank level, adding new plants, reintroducing fish.
Last time I did it it took about 6 hours start to finish but I did have a 1 hour break in the middle for dinner.
Easiest way I find is to remove 2/3 water to a clean (preferably new and well washed) dustbin, transfer fish, add heater and filter. stand lid on top. Leave for the duration.
Do your work on the tank then refill to ½ way add the plants and sort the scape out before putting some of the water from the dustbin back in then float the fish in a large plastic mixing bowl on the top doing the usual water mixing routine. Then let them go and level the water out properly before getting all the equipment running again.
Andy