I have moved tanks a few times and recently upgraded from a 75 to a 90. Here is what I do:
Have 2-3 large rubber maid style tubs on hand.
Mix new water ahead of time and run ph
and heater to get it the same as current water.
On day of transfer do the following:
1. fill one empty tub with water from main.
2. Place fish in tub with water
3. Place rock and other decor in anoter tub and cover with water from main.
4. Place filtration/ph
and heater in tub with fish and tub with rock and inverts.
5. Siphon substrate and discard water just like pwc
prior to doing this save as much of the old water as you can to place in new tank.
6. Set old tank and substrate aside and set up new tank if going in same location.
7. Place new substrate in new tank
8. Remove old sustrate from old tank and place on top of new substrate (you can mix a bit)
9. Place majority of rock and decor in new tank.
10. Put water from rock tub back in new tank as well as some from fish tub leaving enough for fish and at least 1 ph
11. Put new water in new tank.
12. Put filtration and heater in new tanks and let run a few hours until up to temp.
13. Verify reading sg
and temp are close to old readings.and water is fairly clear.
14. Place fish in new tank.
This can take several hours so plan accordingly. When I did my 90g I started at about 11:00 AM and did not place the fish in the new tank until about 9:00PM. If you have any extra filtration (i.e. canister) add it a few days before the planned transfer and then use it for a few days/weeks after the transfer to help compensate. Multiple heaters are helpful as well.
I have never lost a fish using this method.