The alkalinity in a salt mix has almost no bearing whatsoever on the alkalinity in an established reef tank using that mix. With a day or two the alk will be determine by the amount you dose, not the starting values. Water changes generally have only a tiny impact on alkalinity.
For example, if the tank has alk at 8 dKH and the new salt water has alk at 11 dKH, a 10% change only brings the alk to 8.3 dKH. So that boosts alk by 0.3 dKH. Most people add 1-3 dKH per day in typical reef tanks, so that 0.3 dKH boost once in a while is not that significant.
