I don't know if this helps, but this was my experience with switching out gravel substrate with CaribSea Ocean Direct Live Sand Oolite.
I bought a 32G tank second-hand and I would say the person had it several years...towards the end when he sold it to me he did not tank good care of it. BUT, it had established live rock.
Anyway, I did everything in one evening: took the fish out and put in buckets, the same with the corals. I made sure I kept the live-rock wet in newspaper.
I dumped out the old gravel substrate, cleaned out the tank, poured in the Live Sand directly w/o rinsing, added back in the same water with probably 20% new water. Threw in the packet of stabilizer. Waited about 1 hour and literally put everything back into the tank.
My clownfish looked really sad, the female clown literally almost sat at the bottom of the tank. After a couple of hours, the rest of the fish were swimming rapidly in circles around the tank....I suspect there was an ammonia spike/mini-cycle spike (although I did not measure anything). The next morning, all the fish and corals were all back to normal and that white-sand bottom looked awesome.
Lesson learned, I should have rinsed the sand and added in some Bio-Spira bacteria to reduce the mini-cycle. I didn't do that and I think my fish probably suffered some that night....but nothing died and here over 6 months later nothing has died.
If I had to do it again, I probably would keep the fish out of tank in the buckets overnight with an airstone. This would have allowed the tank to settle down/stabilize some before adding the fish back in. In short, if your tank is established you could do everything at the same time in my opinion.