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Unfortunately, what the manager is doing is legal. Pennsylvania, like most every state, is an "at will" employer. In the absence of an employment contract or union agreement, that may give you more rights, an employee can be terminated at any time, for any reason, so long as it is not done for an unlawful reason -your race, religion, sex, age over 40, disability or national origin. Sometimes state laws can add other protected classes, but these are the main ones for discussion purposes.
An employee serves at the will of the employer, and there is no law, state or federal, that says they cannot change your schedule or demand you suddenly work different hours, even if you've had the same schedule for a long time, or they give your hours to someone else. Generally this would be a matter of contract. Now, if the employer's actions are guided by discrimination -e.g., they're changing your schedule because of your race --then you may very well have a legal complaint.
But based on these facts, alone, regrettably, the employer could terminate you in this situation. I'm sorry I can't tell you otherwise.
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