I've read that going from normal salinity to hyposalinity can be done quickly.
However the opposite is not true. Apparently the oxygen levels can drop going up too quickly.
Personally, when i did it, I topped off with DT water (assuming your dt is disease free) to replace evaporation. If a larger water change was needed, i tried to keep about the same salinity that the qt was at.
Also, i made sure that the newly mixed salt water mixed for at least a day or more.