If you are an at will employee, your employer can change the duties of your job. The employer can add more responsibilities, which often happens following downsizing, when every employee must take on the duties of those who were let go. Usually these additional duties do not come with a pay increase. If you are a salaried employee, a pay increase is not legally required. However, if you are paid on an hourly basis, and these additional duties require more than the normal eight-hour day, you may be eligible for overtime pay.
If your employer suddenly assigns additional tasks to your or reassigns you to different tasks, it may not be a negative. Most employers are eager to promote employees but are wary that the employee may not be able to take on additional responsibilities. Your boss may be giving you the extra work as a test to see if you can handle a promotion. It could also be that because of job cuts, your boss needs someone to step up and help get the work done that used be done by those who have left the company.
Either way, the best action is for the employee to take on the extra tasks and do his or her best with these new assignments. Do not assume that you are being punished because you have been given more work. It could be that you are being considered for a different job or that the boss trusts you to help out when the company needs more work done. Once you have mastered these extra tasks, you will be in a better position to discuss a reward, such as a raise, at your annual performance review.
An employer cannot legally restructure jobs as a way to get rid of a certain protected class of people. For example, an employer cannot add the task of lifting one hundred pounds to a job where no lifting has been required in order to get rid of females. Sometimes when there is a restructuring and a particular class of employees is disproportionately affected by those actions, it will take an EEOC complaint and trial to determine if the real reason for the restructuring was actually discrimination.