If an employee who is the owner of a Bluemix DevOps project leaves the company, who can and how do we change the owner of that project to his replacement?
It is possible to have several administrators and to transfer ownership of projects. There is some basic overview of account management for DevOps.
If your scenario is that the employee was the only administrator (like the sole superuser leaving the company) you would need contact Bluemix account management for a solution.