Does salt go bad with age?

[Aquattro]

It depends. I have switched between IO and Kent, but essentially they are the same salt. Many people had terrible experiences switching salt brands this last year, with coral loss or severe bleaching.
I would recommend sticking to one type only, as the whole idea of maintaining a reef is to create stability in the environment. Switching things is not conducive to stability. Would there be any type of logical reason to switch salt brands?