The jobs may start failing when their AD accounts are removed. If the users have left it is better to change the job owner to a different account. To ensure such issues wont happen, it is better to create a generic AD account with only necessary permissions and use it exclusively for this purpose. List down all the jobs which are tied to the users and share it with all the respective project managers and technical architects and get a approval . Decide on a common user account and create new one .