After a new version of Kubernetes has been released and when Container Engine for Kubernetes supports the new version, you can use Container Engine for Kubernetes to upgrade master nodes running older versions of Kubernetes. Because Container Engine for Kubernetes distributes the Kubernetes Control Plane on multiple Oracle-managed master nodes (distributed across different availability domains in a region where supported) to ensure high availability, you're able to upgrade the Kubernetes version running on master nodes with zero downtime.
Having upgraded master nodes to a new version of Kubernetes, you can subsequently create new node pools running the newer version. Alternatively, you can continue to create new node pools that will run older versions of Kubernetes (providing those older versions are compatible with the Kubernetes version running on the master nodes).
Note that you upgrade master nodes by performing an 'in-place' upgrade, but you upgrade worker nodes by performing an 'out-of-place' upgrade. To upgrade the version of Kubernetes running on worker nodes in a node pool, you replace the original node pool with a new node pool that has new worker nodes running the appropriate Kubernetes version. Having 'drained' existing worker nodes in the original node pool to prevent new pods starting and to delete existing pods, you can then delete the original node pool.