Changing single in-house IT person, what to do?

Hello,

We’re looking for advise on best practices when changing a single in-house IT person with a new person.

Obviously, changing passwords is a first, but what’s the best thing to do if the relationship is ending on bad terms and the passwords to various devices are not made available or are left undocumented.

Any checklists, guides, resources that could help create a master plan would be fantastic.

Thank you all in advance.

Brian