As everyone else said, typically switching salt brands isn't a huge deal. I wouldn't mix the two salts together when making up your water change water however. Just do your water change like normal but use the new salt.
I had the same question. Before knowing the differences in salt brands, I purchased the 200 gal Instant Ocean mix. The LFS charged me 69.99 + tax . Then I found out that it contained heavy metals . I did not want to put it into my tank, so over 70 bucks down the drain. Now I guess, I can start my cycle with Instant Ocean and after it is all consumed, I will change brands.
All salt mixes are going to have some degree of "garbage" in them. This is because NSW
has never been exactly duplicated no matter what the manufacturer's claim. I would use the IO bucket you paid for and if you don't care for it, switch to something else after you use it up. Hate to see you throw money out the window.
I used IO when I first set up my tank. I didn't care for it because of the chronically low Alk
levels. I switched to Reef Crystals hoping that it would be better, but wrong again!! I just purchased a bucket of Oceanic with hopes that the Alk
level in that will be higher. From what I have been hearing, Oceanic is supposed to be one of the "best" salts around. Who knows...