well..if I do a complete gutting of plants, rock, driftwood, I tend to kick up lots of mulm...also would scare the hell out of the fish with no where to hide.
as a result, with MAJOR rescaping, I move all the fish out. Do all the dirty work of re-scaping, do a BIG water change...let things settle out and temps to re-equate...then return the fish. Done this 2x with no ill effects.
Almost have to do at least 80% wc with everything you stir up. If I have too much still floating afterward, I sometimes do a second 50% wc. The fish will do OK either way, as long as you can control temperature changes pretty close. If not, there's always a bucket handy for me to put them in.
I play it by ear, the substrate and extent of the roots can make differences. As long as I maintain consistency and don't make dumb mistakes (e.g., forgetting dechlor or something like that), I'm good.
I just did an aquascaping revamp yesterday, so I remember clearly what I did:
1) Rip out all the plants and place in a plastic tray or bucket
2) Drain the water until only a couple inches remain
3) Fish out all the fish (it is much easier to catch them and much less stressful than sloshing a net through a tank to chase them down)
4) Clean glass
5) Drain out the rest of the water
6) Take hardscaping material (rock and wood) and begin to position it in the aquarium
7) Fill the tank half way full with water
8.) Plant specimen plants (Blyxa, Anubias, swords, crypts) first
9) Plant main foreground plant (glosso in my case) and then accent foreground plant (E. tenellus v micro in this case)
10) Plant red plants
11) Plant green stem plants
12) Fill up the tank with dechlorinated water
13) Turn on filter. Let the water circulate, the temperature equalize for an hour
14) Put all the fish back in
15) Leave the lights off/dim the lights on the first day after rescape
This method has always worked for me. I have some really sensitive fish (rummynoses, chocolate gouramis). They know the drill by heart (I've done this three times so far this year). Still eating and swimming around happily, coming up to the glass the next day to greet me when I come back from classes (to beg for food). 8)
Oops, skipped a step. I drain all the water and siphon off as much mulm as I can. I fill it half way up again, then siphon off the mulm a second time. Repeat another time (total of three mulm siphoning sessions).
i used to change my aquascape heaps. unless i was replacing the substrate or something, i always leave the fish in there and all seem healthy. i'd syphon till i have about a couple inches of water and do wat i have to do. i have some fairly hardy fish (eg angels, red tail shark, clown loaches, SAEs, neon gouramis, etc.) i've never lost a fish doing it this way - though i nearly lost the clown loaches hiding in drift wood which i thought wasn't even hollow. those guys squeeze into the tiniest of holes.