They don't have to provide notice except to tell you about the change happening as of that day, but they can't take away what was already accrued. But they weren't required to provide you any notice and I am not sure how notice really would have helped you. I think you assumed you would get sick leave on top of your vacation time (which is what the government would have liked employers to do and they made it sound like a huge employee benefit they got for you by law), but not all employers can afford to give another week of PTO additionally to what was in place. And since there is no requirement for vacation time, the employer made the best choice from their perspective.
Starting on that day in 2015, the employer chose to replace part of your vacation accrual with the government mandated sick day accrual. There is nothing illegal about doing so. The government can try to force employers to give more benefits, but it doesn't always work out that way. In the end the employer has to do what is best for their company under any mandated laws and the employee can choose to look for other employment if you are unhappy about the new setup.

