It shouldn't be to big of a problem. I personally don't like changing salts simple because I never know what each different levels will be IE: CA, MG, ALK, etc. I use IO and it's been by far the most stable salt mix. Never used Aqua Medic salt so cannot help much there.
If I had to change though I would do it over a few water changes. Slowly introduce the new salt in while monitoring the levels elements of the mix. Chances are you will have to change around the amount of buffers you'll need. IE: if you add 1 teaspoon of ALK to fresh SW with the old salt you might need to add 2 teaspoons to the new mix.
