I'm not a saltwater keeper but in my very humble opinion I believe you just need to wash most everything with hot water to get all the build up off, and put in new filter media.I say most because I would not use any of the crushed coral or sand form the previous tank, those things have a tendency to keep the Ph up on the high end. And now since you are going freshwater you will not need any of those fancy bulbs needed for a
SW tank. Trade up or sell your old set ... think of the money you can save. - hope this helps