When I started this hobby, I started with Red Sea regular salt. It was ok...nothing really one way or another to say about it. When I started keeping corals, I switched to IO since that seemed to be what everyone was using. It was inexpensive and great...until my 40g SPS dominant tank started having problems.
Two different Montipora species were dying, and I couldn't figure out why (M. spongodes/confusa/whatever and M. peltiformis). The rest of my corals were looking pretty rough, but not terrible. I had lost all blue coloration as well. I was posting everywhere trying to find a solution. All chemicals checked out...but nothing. After extensive research, I found the culprit by the time it was too late. I was low in potassium. At that point in time, very few studies had been performed as to the effects of low potassium. There also were no suppliers of a pure potassium supplement available (until about a month after this all happened.)
Since I had regularly performed 10% weekly water changes up to that point (and several larger ones sporatically), I decided Instant Ocean was no longer the salt choice for me. I switched to Reef Crystals for a while...and for whatever reason, I decided to try RSCP. I'm definitely an addict of RSCP now.
Even with RSCP, I'm now up to 150ml of each additive (2-part solution) per day. With Instant Ocean, it was next to impossible to supplement enough. Keep in mind, I have a 40g main tank + 20g sump...with about 90# of LR and 50# LS. By the time you calculate how much water is actually in my tank, we're guessing about maybe 30g. I'm also keeping around 70 SPS mini-colonies.
My point is, many different salts are great and have their advantages. IO is good for fish-only or low-coral load tanks (or if you keep SPS and find it less expensive to just dose a lot.) After losing two of my favorite corals, I felt that I'd rather pay more money and have healthier corals in the long run.
To switch salts, about halfway through a bag or bucket, I would add the contents of the other salt. Make saltwater as usual. By the time the bucket is used up, I feel comfortable enough using the new salt. Other people gradually increase the amount of new salt in the saltwater mix...and some people don't do anything but just switch immediately. There doesn't seem to be a hard & fast rule.