I don't think there's any legal problem with you keeping a plate from your old car.

I don't think the linked document applies to your case. PennDot will allow you to return old plates so they can't be used fraudulently, but there's not many cases where plate return is required. I believed it was only for revocation of registration, or if you choose to cancel car insurance.

Other states are neurotic about returning plates. Don't let stories from outside the commonwealth spook you.