If you must do a water change it is important to add back to correct amount of salt/per gallons changed out. You need to keep the salt level up for the entire treatment period.
Edit: Correction - if your water change is after 1 week of salt treatment you do not need to add more back. Someone please chime in if I am not correct. I initially added 2 tsp per gallon, but added it very slowly. If your fish are ok after adding it all at once, you should be ok. It is my understanding that it takes 2 weeks for the Ich to run its course.....so keep the temp up and good luck.