Remove Specific Taxonomy Term when Associated value in field is gone?

How do I remove a specific taxonomy term when the associated value in a CCK field is gone.
E.g. I have the field teacher. It's current value is Mr. Brown. It's new value will be Mrs. Violet.
The associated taxonomy terms are Ken Brown and Kendra Violet, respectively.

If I change the value from Mr. Brown to Mrs. Violet, I want to remove the taxonomy term Ken Brown and add the taxonomy term Kendra Violet.
However I need to know that the previous value was Mr. Brown in order to remove the taxonomy term Ken Brown. How do I do this?

I already know how to add a taxonomy term based in the associated new value selected in a field.

Thanks,
Tom