Yes, your manager may do this, as long as 1) you do not have an employment contract to the contrary, setting your rate; and 2) you are notified prior to the meeting (i.e. the change cannot be retroactive, after you attend the meeting). When there is no employment contract, employers may change rates or pay at will, and may have different rates for the same employee, depending on the duties or work she or he is doing at the time.