Promote - how can I promote a person instead of simply the assignment?

Summary:

Currently when promoting a person by selecting one of their many assignments, the promotion is only shown on the assignment which was selected. Furthermore, different promotions can be given simultaneously under different assignments. This is less than ideal for our client. We are trying to promote the person as a whole, across all assignments to reflect the promotion.

As an example, you may draw a parallel to being promoted from Silver tier to Gold tier as a user across the entire system. We have set it up so that salary, benefits, etc are linked to these supposed tiers instead of each individual assignment.