Can I Convert A Salt Water Aquarium To A Fresh Water Aquarium?

Yes you can but NEVER EVER use SOAP in a fish tank!!!
You can dump the waters and throw away the corals and rock. You’ll never get the salt out of those.
Plain house hold bleach will do to clean the tank (chlorine).
Dump it again and again. When it’s clean, fill it, treat it with chlorine remover and let it set for no less than 2 weeks before adding new fish.

its simple enough. remove the water and rince everything.
you will have to replace the substrate for a fresh water aquarium gravel and your protein skimmer won’t work in a fresh water setup but heater,filter etc will do just fine. they are designed to resist the corrosive effects of salt so will be fine in fresh.
a fresh tank is NOT harder than a salt tank its just that people tend to get much better equipment (filters etc..) for a salt tank because replacing salt water costs money (fresh water is free) and salt fish on average are more expensive.The better the equipment usually the better it works, you will usually see that high end freshwater filters are also rated for salt,using the other way around just means you’ve got top end fresh filters
Choose your fish in advance and considering the size of your tank either an african cichlid set-up or a planted south american community tank would look great.
almost forgot ,depending if you have actinic lighting you may have to change the bulbs in your tank
heavy mineral build ups i usually scrape off with a razor blade.
its a big job but it will look beautiful when complete

Drain and clean the aquarium and all of it’s contents. If you are interested in African Rift Lake cichlids you can use the coral and other items for your set up. If you have live rock that can’t be used though since you weren’t talking about a reef tank I assume it’s a basic salt water aquarium with crushed coral and maybe a few larger decorative pieces. I’ve used crushed coral for my Tanganykan cichlid aquariums for years and it helps tremendously. I know people that use crushed coral or dolomite for Malawian cichlids as well. I personally usually just use it for my Tanganykans though I think it would benefit all Rift Lake cichlids. The crushed coral will help keep the ph and hardness up.