Everything I've read has said PO Box is fine and for change of address you just have to send in a Change Form and $10.
From the FAQ:
Question:
What do I do if the address on my photo ID does not match the mailing address provided on the application?
Answer:
The MMP does not require the address on the photo ID to match the address included on the application, provided it is a correct address located in Michigan.
Question:
Do I have to tell the MMP if I change my mailing address or change my designated primary caregiver?
Answer:
Yes.
The MMP only accepts written changes regarding the patient and/or their designated caregiver (name, address, change of designated primary caregiver).
If it is time for the patient to renew their registration, all changes can be made on the renewal application. If it is more than 30 days before your card is due to expire, you will need to submit a change form. There is a $10.00 fee for issuance of a new registry ID card. Your new card will reflect the changes you request. You will be protected from civil and criminal penalties for these changes once you receive your new registry ID card.