Once generated, do the QR codes for the entry pass ever change?

As long as the policy conditions don’t change the code generated with the entry pass will not change. 

When a user’s entry pass is past the expiration date, the status of the entry pass changes in app to show “Expired” state.  The user can no longer view a QR code on expired entry passes; they are simply presented an option to renew or create an entry pass.