The registration certificate part I has replaced the vehicle registration document. The person entered in the registration certificate Part I is the keeper of the corresponding vehicle. The term "keeper" originates from § 833 BGB. According to this, the holder is the person who uses the vehicle for their own account (i.e. pays the costs and can actually dispose of the vehicle; this is often not the owner of the vehicle. The registration certificate part II is therefore not proof of ownership, but ultimately only facilitates the registration process, as it is assumed that the person holding the registration certificate part II is authorized to dispose of the vehicle.
There is no general obligation to exchange the old vehicle registration certificate for the new registration certificate part II. A new vehicle registration certificate is automatically exchanged if a new one has to be issued anyway (e.g. because it has been lost, regardless of whether the old vehicle registration certificate is still available or if a new registration certificate Part I has to be issued.

Part I registration certificates (vehicle registration certificates) that have been invalidated will be returned to the owner after the vehicle has been taken out of service.