You can bring the salininity up in 24 hrs. or less. Its the same thing as acclimating critters in a bag.

I would recommend doing small water changes with saturated salt water, or just doing it slwoly by using salt water for makeup water.

I would suggest testing the salifert and seachem alkalinity kits side by side and perhaps run the calibration standard as well. I wouldn't be surprised if your alkalinity wasn't making some pretty good swings due to the other changes in your tank. Mine can change a couple by a couple of dKH over a few days. That's why I dose buffer every 3 days.