When changing salt brands, it's best to start small and gradually work towards a 100% switch. An instant switch could throw the whole tank off and cause a crash. If you do a search here you can find horror stories of people that had just this happen. So I would start by just adding a little bit of the new salt to a solution of your old salt for the first water change, and gradually increase the concentration of the new salt brand. I'd say within 5-6 water changes (don't hold me to that number......*not responsible clause*) you should be able to switch over completely.
Did Dr. F&S get rid of their extra charge for shipping salt? That might be a good way to go if they did.
If you ask Chris, he had a problem going to Tropic Marin. And supposedly other people had the same problem, so I'd stay away from that brand all together.