Its hard, but definitly worth it. Problem
Is its also very expensive. You will need a few hudred dollar lights, a nice skimmer, sump, (you can keep your filtration), overflow, new sand, etc etc. lookin at easily 700-1K, and thats before you get fish and livestock.
Definitly possible though. Rehome fish and everything, and steralize tank, start saltwater cycle! Lol just need all the equipment first
Edit: if you do a fowler tank you dont need bice lights so its much cheaper
also, the price rangs i estimated is if you get the right equpiment and dont take shortcuts.