Starting a New Tank

Hi I have not posted much but am now going full force into tank . I have a Cichlid Tank 55gal and a 39 gal Salt but want to go full saltwater . I am looking for someone to take my freshwater fish , my question is when I get my freshwater tank empty and start up salt How long should I let it set and will I be able to transfer my fish from my 39 gal. to the 55 gal because I was told that if I transfer them they would die is this true

 

What filtration/live rock/protein skimmer (if any) do you have on the 39? What fish do you have? Do you have a sand bed? Do you have corals?

In general, if you save the filtration media, as much of the water as possible, the sand and rock then you should have minimal issues in moving up to a bigger tank provided you don't add any additional bioload.