How can I create a Dashboard to display only those domain User Accounts for which the contents of a specific AD attribute has changed?

Specifically, if an AD user account attribute "employeeType" changes from "NULL" to "Contractor", how can I detect/filter for that? Ideally I'd make a Dashboard to display only the User Accounts for which that attribute changed from NULL to Contractor (or something other than NULL) over the previous week. I have Active Directory feeds already set up, so I already receive historical data (snapshots of AD) that contain the data I need (i.e. the 'before' value and 'after' value of the attribute).