I'm not aware of any reason you would need to surrender a license plate back to the state unless your insurance lapses and then surrender of your plate for three months is one of the penalty provisions.
Unless you have been directed to do so you are under no obligation to return your old plates.
Nothing, unless you had an order from PennDOT to turn them over. Surely you have seen bars or restaurants adorned with license plates from all over the country. The state only wants the plate back if your registration was suspended, and they want it to ensure you serve the suspension because you can't drive the car without it.
This is not intended as individual legal advice and there is no attorney client relationship established by this answer. It is advisable that you seek individualized legal assistance. This is not a substitute for hiring an attorney.
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