How to re-assign user assignments
When somebody leaves, or changes job, their assignments are scattered across runs in several departments. Reassigning them by hand means opening every run and hoping you found them all. There are two tools, and which you need depends on the scale.
Reassigning within one run
Open the run, use the More menu at the top right and choose the assignments page. It lists every step in the run with its own assignee, so you can move several steps to somebody else in one pass. See Assigning steps in a run.
Reassigning across every run
For somebody leaving, one run at a time is the wrong tool. Use the account-level reassignment to transfer that person's assignments to another person, or spread them across several people, across runs and departments in one action.
This is the offboarding step people forget, and the symptom is familiar: work quietly assigned to somebody who no longer works there, showing as nobody's problem until it is late.
Do this before removing the person
Move the assignments first, then deactivate the account. Doing it the other way round leaves orphaned work that is harder to find, because you can no longer filter by the person who had it.
Worth checking at the same time: whether they were the only person in a role. A role with nobody in it silently stops resolving to anyone on future runs. See Role based assignments.
A more durable fix
If reassignment is a regular chore, the underlying cause is usually workflows that assign steps to named people rather than to roles. A workflow assigned to roles survives people joining and leaving without anyone editing it.
Video walkthrough
This video walks through transferring one person's assignments to somebody else.