What can happen if I change a patient's address in the system without patients permission?
Response: I don't think that changing, correcting, or updating a patients address in the providers system necessarily requires their permission. As long as you have a credible basis for doing so and the new address wasn't obtained illegally or unethically.
Creditors and others businesses will update a persons address frequently. I think all you have to do to get a person's forwarding address through the post office is to place "Return Service Requested" above the patient's address. When the letter is returned, it will many times have the person's forwarding address printed on a yellow sticker by the postal service.
I believe this is based on the Post Office Mailing Standards of the United States Postal Service Domestic Mail Manual. Section 507 is titled "Treatment of Undeliverable First-Class Mail, First-Class Package Service and Priority Mail".