Changing salt mix

I recently bought a batch of instant ocean salt mix. It came with a 2.5 gallon jug and 10 packs of salt. I got it very cheap from a LFS. I normally use reef crystals but as I said I got this cheap. I instantly noticed a big drop in my calcium levels. Corals hated it and so needless to say we won't be using that again no matter how cheap it's offered. So I am now thinking about changing to Red sea pro salt or H2 ocean. I have been told to do heavy water changes to switch over the salt water. I am worried about the affects of this on my reef system. I have mostly soft corals and LPS. This does not seem like a good idea to do large wc to switch over salt mixes. I would rather not go back to reef crystals because I always get a weird film on the water when I make it up. It also takes for ever to mix thoroughly. I use R/O water and Have a 50 gallon show tank with a 30 gallon sump. I use a Max spec LED light fixture. All levels are within normal range before the Instant Ocean debacle. And yes I should have known better. So I would like your Ideas on how to go about changing over my salt. I normally do a 5 gallon WC every other week. I was thinking of going to 2.5 gallons ever week. Also feel free on giving your thoughts on salt mixes.