I was involved in a couple of Change Management projects in Accenture Strategy "Talent & Organization" practice, therefore I can give me my point of view.
HR due dilligence would probably refers to what is called "right sizing", which basically means asses the right size of the company/branch/function/division in terms of number of employees. In other words, what is the minimum number of emplyees that would be required, on average, to perform what needs to be done (all the activities in scope).
To do that, you usuallt want to look at
- AS IS number and type of activities in scope (eg. 16 activites
- AS IS time spent to do these activities per week ( 10 hours avg per activity per week)
- AS IS number of FTE required, based on the previous variables (eg. 160 hours per week / 40 hours per FTE per week = 4 FTE required
Then you want to compare it with the actual number of employee that they are using. From the comparison you can underline where and why they have efficiencies / inefficiencies.
You can also try to find other efficiences and draw a TO BE situation (maybe the activities can be reduced to 12 per week aggregating some of them, or maybe you can automate some parts of these activities and reduce the average time needed).
Change Management can be related with either the "soft" elements of an organization (skills, behaviours) or the "hard" parts (organization charts, processes...). Can also be related with both at the same time. You always start with an assesment of the AS IS situation and you compare it with the desired TO BE situation (client desired fine tuned by consultants analysis and recommendations), and then implement a strategy (series of "actionable actions") to fill the GAP.
These are just very general frameworks, but they fit well for general and open cases.