Prevent users from inadvertently un-tagging terms they don't have CREATE privs for?

Drupal 5.9, MySQL

Suppose I have two roles, HR admin and IT admin. I also have a "node category" vocabulary that looks something like the following:

HR Documents
- Payroll
- Workers' Comp
IT
- Support policies
- Training

Via TAC, the HR admin hrole as create and update privs on HR Documents and its child terms, and the IT admin has create and update privs on IT and its child terms.

Say I have a node that is tagged as both HR and IT because it is applicable to both. If an IT admin goes to update the node, that person sees only the IT terms listed in the taxonomy box--as it should be--but after the node is saved, the node is no longer tagged for HR, even if the IT admin didn't touch the taxonomy box at all.

Instead, I need to have it so that when the IT admin updates the node, it does not affect the HR tags--neither adding nor removing them--and when the HR admin updates the node, it doesn't affect the IT tags.

Any suggestions on how to accomplish this?