Although I'm sure others may disagree with me ...
Ya, I can see how this is a concern for many, but doing a 50% change with 15dKH water into 7dKH will only net you 11dKH.
Although this may cause some shock to your corals, it won't kill anything healthy, IMHO ( the amount of Alk, Ca and Mag in the tank and the NSW are each devided by 2 )
These numbers, although a bit hi for the liking of some reefers, are not detrimental if they stay balanced with each other, and will eventually drop to your more comfortable levels
How many corals you have in your system, and coralline, will determine this drop rate
Many reefers have had to perform large water changes at some time, and I'll bet most would tell you that it, in itself, is not a major concern
I wouldn't, in your shoes, be overly worried about having to perform the occasional 25 - 50% WC in an emerg situation
It's just too much to bother worrying about, but is a great question
You may find ( until you have a large amount of corals and need to start dosing ) that you can control the exact params you'd like to keep by performing WCs