Sorry to hear about this GSP. Truly unfortunate. I have switched tanks more than most, {but thats another story,
}. Seldom any acro or other sps losses. Now that I have said that, my stuff is moving from my 20g to my 30g this weekend, so hope I never jinxed myself.
Cant see salt mix being a problem. Lots of times, I have mixed a major cganhe for only a couple hrs. no problem. Most of my tank changes have been like yours. Part old water and part new water. Have not done sand changes in awile but when we moved Mike,s from his 120 to his 150, we moved most of his sandbed, which he did rinse well prior to putting it in the new tank. And we left the sand over night before moving more, so not sure if that helped or not.
A few years back, the one time I did have a problem and lost many sps and clams on a tank move, was an alkalinity issue with then new water. How many of us measure the alk. of the new water? For sure back then I never. Do now though.
Other than that I,m at a loss.