I have seen some customers who have the same issue with high turnovers but they wanted to keep the ability for their replacements to be able to continue the work seamlessly. i.e. access to all work items and their history.
From what I recall, they opted for a dual ID system where they were assigned a work ID in AD in addition to their normal accounts. So in theory, the work ID would be passed to another person if the current employee leaves the company.
This could also apply to an implementation using SQLUM where the work IDs are stored in the SQLUM database.
Does this make sense?