Saltwater upgrading isn't much different than freshwater upgrading. Just reuse as much old water as you can and don't add too much in the way of new live sand or new uncured live rock. Try to parm match the new water with the old and you should be fine. Also be mindful of the lighting differences if you're changing lighting types. Don't want to burn the corals.
No trees were harmed in the writing of this post. However, several electrons were GREATLY inconvenienced.
"If nobody makes you do it, it counts as fun." --Hobbes the Tiger