The Process Owner or Expert has been changed through the Summary section of the Process, however the Process Approval notification has gone to the previous Owner or Expert.

The Process needs to be approved by the previous Owner or Expert and then Published.  

Once the Process has been published the new Owner or Expert will be required to approve all future required Process approvals.

An alternative option to bypass the approvals Process is for a Promaster to use the Global Replace by transferring the ownership through the Admin > User page.  

Note: Using this option the Process will still be published, so it will Publish any unpublished changes for the Process.