1) Going forward, your employer may change how you are paid from hourly to salaried at will, without notice or your agreement, unless you had an employment contract to the contrary.2) The change is only effective going forward from when you were notified of it. You should be paid for all work done up to when  the change was made at the then agreed-upon rate or terms, so the employer may not simply deprive you of 20 hours  overtime.3) Also, even if you are salaried, you may still be eligible for overtime--that is, being salaried does not, by itself, prevent you from earning overtime when you work more than 40 hours in a week. To be exempt from overtime, not only must you be paid on an hourly basis, but your job/duties/authority must also meet one or more of the overtime exemptions. You can find these on the U.S. Department of Labor website and should compare them to your job--the main ones to consider are the administrative employee, professional employee, and executive (managerial) exemptions. If  you don't meet one or another of the exemptions, then even if you are normally paid a salary, you must receive extra compensation when working more than 40 hours in a week.It can be difficult to calculate overtime for a non-exempt salaried employee. If  you think you may be misclassified, speak with an employment law attorney: he or she can confirm if you have been, help you understand what you may be entitled to, and take action (including for the lost 20 hours of overtime) if appropriate.