RE: GP Workflow 2.0 - using workflow hierarchy/role for approvers vs Active Directory hierarchy
Great question!
From what I understand, yes, this will move automatically to the individual assigned in Active Directory.. based on the "start from" and "assign to" field used in the workflow user selection window.
I have not tested this, but I feel like the GP Workflow Manager (identified in the workflow type setup) may need to get involved, if the PO is submitted, and THEN the AD manager is changed... not 100% on that part but just a hunch. Anyway, based on your question... if the AD manager is changed and then the document is submitted, it would look for the values assigned in the 'assign to' field and automatically pickup the new AD user as the approver.
Here is a link to a reference that I frequently use, in case you have not seen it.
https://community.dynamics.com/gp/b/dynamicsgp/archive/2014/10/08/workflow-2-0-39-s-workflow-user-setup-window-options-part-2-hierarchy
Also, you can easily set this up in a test company and try it using the use case discussed here. Once verified, a few clicks can copy it to your production company(s).