Federal equal employment opportunity laws do not prohibit employers from changing their paid sick leave policy if it is done in a manner that does not discriminate between employees because of race, sex, age (40 and over), color, religion, national origin, disability, or veteran status.  Be sure also to consult state and local laws.
In addition, you should consider that if your workforce is represented by a labor union and the collective bargaining agreement covers sick leave policies, you may be limited in either the manner in which you change the policy or the manner of the changes themselves because the collective bargaining agreement would be controlling. In a workplace without a collective bargaining agreement, employees may have a contractual right to any accrued sick leave, but not future leave.
Your sick leave policy also has to follow the requirements of the FMLA (if your employees are covered by the Act), and it needs to be consistent with federal workplace anti-discrimination laws, such as the Americans with Disabilities Act (ADA). (See the U.S. Department of Labor, Wage and Hour Division or call 1-866-487-9243 for additional information on FMLA.  See the U.S. Equal Employment Opportunity Commission or call 1-800-669-4000 if you have questions on ADA.)
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