This is totally do-able but will be a bit of a pain in the butt, especially if you want to use the 15g's substrate in the 90. It's really just a matter of figuring out what you want out of the 90, purchasing everything (tank, stand, heater, decor, substrate, light, etc), and then blocking off a day to dismantle the 15g tank and put together the 90. I recently did this moving from a 20g to a 40g and it was annoying but not incredibly difficult. Fish went in a pot, plants went in shallow baking dishes covered with plastic wrap, filter media went in a bowl of water... though maybe check with the person who owns all your dishes if you go this route. If you're itching for a bigger tank and are going to upgrade some day anyway and are sure you can deal with it why not just do it now? Good luck!