Essjay is absolutely right.
I did this same exact upgrade. The only difference was that I did not use the same substrate.
I upgraded my 55 to a 90 and put it into the same spot.
You'll need a few buckets.Some for fish, decorations, and filter media. The most important thing you'll need to do is keep your filter media submerged in tank water. Decorations too if you have extra buckets.
I kept 2 10 gallon bucket filled with water and fish. That was the only water from the old tank that I added back to the tank. It also gave me the advantage of getting the fish back into the 90 without acclimating before I began to fill with fresh water.
So for the steps.
Take down the tank and separate all of your existing things in buckets. Keep media and decorations or plants submerged. You shouldn't need an airstone for the fish provided you work quickly. If you can set one up I would absolutely do that.
Old tank out, new tank in.
Add gravel and decorations. Start getting filter, heater and decorations in place. Add fish and tank water (granted you have kept enough for the fish to swim)
Fill tank up and get your filter, heater and everything else running.
What you'll have in the end is equal to a very large water change.
Continue to monitor ammonia especially to be sure that it doesn't cycle. It should not cycle.
If your filter is powerful enough to handle to bio load on a 55, it'll have an even easier time with the same stocking on a 90. Don't add more fish until you've been certain that your tank does not cycle.
I've done this multiple times when moving, switching, and upgrading tanks. Maybe someone has done it a tad differently than I have, but I have had 100% success every time.
Good luck! Oh, and post us some before and after pictures!