Keep it the same, then if you need to, transfer the fish/livestock, then change the salinity level. Otherwise, you will shock your fish. Not good, if it is already sick.
Also, if you are bringing in new livestock, you should do water exchanges from your tank to the bag, then take water out of the bag & discard it. Do this a bit at a time, to ensure that your livestock doesn't get a salinity/ph shock.