It depends on how the tank is currently setup as to how difficult it will be, but it definitely can be done. Most SW tanks rely on live rock for the bio-filtration so that would probably need to be replaced with different bio-media. The protein skimmer won't be needed either. Other than that the tank would just need a good cleaning to remove all the salt creep and coraline algae. If it was a reef tank then the lights will probably be overkill for a FW setup as well, so you could sell them off to buy a lower intensity light and new filter.