Lots of upside to doing the move with some things to work out along the way. What you want the large tank to become is one biggie. Do you want it fully planted or just some plants as an "upgrade"? Either will work but require a bit of different action. I worked into planted from the larger tanks kind of by accident and did not really find any large problems. Large tanks are so much more stable and easy to deal with than small. You may want to develop easier ways to change water. Larger/more filters if you have larger fish but that depends on the fish not the gallons of water involved. If you kept the same fish that you have in a 15 in a 90, things are obviously much easier with the extra dilution of pollution. A bigger heater is obvious but then you may also find you can get away with no heater for a day or so in a large tank before it cools too much. Just lots of small things. Put some plants in the dirt you have and pots and see how it goes?