Is it legal to change pay structure without notice?

my compensation is salary + commission. (minimal salary) we are not advised of our budget (the amount we are paid commission on for any given month) until after the period begins, and it is arbitrarily determined. that is to say, we as a sales staff are not informed of an annual percentage (increase)to meet each month. instead, each salesperson is given a goal based on their prior 30 days of sales. at the beginning of any given period, we can be informed that we have to produce 50% more than we did the previous year and have less than 30 days to accomplish, while other reps have 0% increase over prior year. i wonder if this is legal or if the salary is just to deflect the fact that our commissions are manipulated to avoid paying the high performers what they have, effectively, earned.