When a user leaves your organization, your IT admin will typically suspend or delete the user's account in your identity provider dashboard. As part of that workflow, the admin can usually choose to transfer data and permissions for the departing user to another user in your organization; this includes permission for access to Coda docs.
If your organization is connected to Google Drive, your GSuite Admin can handle this on your behalf. See this Google help article for more information: https://support.google.com/a/answer/1247799
If your organization is not connected to Google Drive, then one of your Organization Admins can change doc ownership from the Organization Settings panel.