how much notice does an employer have to give an employee when changing their wages

An employer cannot change the wage rate of an employee for work done in the past.  However, absent a written agreement to the contrary, there is no minimum amount of notice required to change the wage rate for work not yet done.  Thus for example, if an employee has regularly made $10.00 per hour, his employer could legally chage his wage rate to $9.00 per hour (or $12.00 per hour) at the beginning of his next shift or in the middle of the shift as to the hours not yet worked.