I did manage to identify and replace one of them, but it wasn't easy.
The first problem was that the Provisioner never stood still long enough for me to assign them to something.
Eventually I settled for just sending them to another Settlement to get them away from their assigned duty.
Which seemed to remove the supply line, allowing me to reassign my http://steamcommunity.com/sharedfiles/filedetails/?id=650942131.
http://steamcommunity.com/sharedfiles/filedetails/?id=650942257.
Since he went on the correct path and showed up in the Mechanist's Lair an hour later, I assume the process worked.
However, it won't be so easy with Hub Locations, such as http://steamcommunity.com/sharedfiles/filedetails/?id=650942384.
The number of Provisioners coming and going means identifying them individually is incredibly difficult!
Not to mention the fact that my reclaiming of all the Settlements in the area means there are several lines to reassign.
Finding each individual Provisioner would be nearly impossible, as their time at each settlement is limited, they show up, walk around for a bit and are gone again.
I think this system needs to be revamped, that you don't need to be physically near or even see the NPC you are reassigning.
Just locate them on a list and do a drag/drop reassignment,