It's often the case users use the Make Copy on POs as this saves them time. However, there is a field they often forgot to update and I was wondering if it's possible to unset this field when the 'Make Copy' is being used inside a WF action?

Yes you can set the context to Copy and the WF will execute in this context. Or just start the first state of your WF by clearing that field upon entry and every PO will under that field just once when it first enters the WF

Ace, thanks