PA state employment law, employee benefits, can employer prevent carryover of vacation

Assuming that you are not a unionized employee (subject to a collective bargaining agreement), the answer is generally yes. You have no absolute "right" to vacation time, or when it may or may not be used. Paid vacation benefits are discretionary with each employer. That said, the new policy cannot take away any vacation that you may have already earned, but I don't see that as a factor based upon the information provided.