Take as much water, rocks, filters, gravel as you can from the old tank to the new. If you were able to take all the water to the new tank and all the gravel and such and fill with treated tap water it would be akin to doing a 50% water change on the 90 gallon tank.
Go to Home Depot/Lowes/True Value/Etc and by a bunch of 5 gallon gas cans. Fill those with water. Easy to transport and easy to pour. Start by draining some water from the top of the tank to a cooler or plastic bags or whatever you are putting your fish into for transport. Take it from the top as you don't want to stir up the crap that is in the substrate.
Then start filling the 5 gallon gas cans. Again from the top. You don't want to transport all the much that is in the crevices and crannies of the old tank.
Once you get all the water out you can without having to pull rocks and mess with the gravel then go ahead and just pull those ornaments/rocks/wood out and get them ready to go. You should still have an inch or two of water in the tank. Leave it. Don't syphon it off. If you syphon it off, all the muck that is stuck in the places you couldn't put the syphon hose is coming with you.
Then I would get the biggest sieve you can find. Have two buckets waiting. One bucket is for the nasty water. The other is for the gravel. Scoop out gravel with a half gallon plastic water pitcher and dump the gravel into the sieve and let the water run down into the bucket. Then dump the gravel into a different bucket. Repeat till you have as much of the gravel as you intend to take with you in buckets. You are still going to bring some muck with you, but it will settle and/or be filtered out in an hour or so after setting up the new tank.
Load everything in the car and head to the new tank. This includes all of the filters even the ones from the old tank. Drop in the gravel and the water. Add more treated water to get it up to a close to the top. Kick on the filters. Check the temp. Once the temp is good, put the fish in.
Even though I doubt you will be keeping the other filters on the tank long term, they will greatly help the tank to stabilize.
You may lose a fish or two just because the process is so traumatic, but the more you bring from the old tank, the better.