Salinity
Yes, it is OK to do a water change a few days later. Just make sure you mix the water thoroughly for at least 24 hours. I use a pump with an air fractioning impeller (Needle wheel impeller.) that is drawing in air. After using this pump to mix my water for 24 hours, my water will be thoroughly oxygenated. This will raise my redox level in my mixed water. But a good power head will mix the water pretty well also. Make sure the water temperature is the same as what is in the tank. During the winter, you may need to put a heater in your mixing bucket. for the 24 hours you are mixing the salt and water. Once you have the salinty and the tempurture where it needs to be, and after the 24 hour mixing time, you may change the water.
I have used this method for years in my reef tank. Sometimes, I have changed the water as many as three times in a week, with no ill effects.