Ok I can test the subpacks down to the cell level, so the bad cells can be identified, now how do we get the cell replaced. Grabbing the stick with plastic strips in the milling machine vice, I milled off the welds being careful to not machine into the stainless steel case. Once the welds were removed, the cup just slid off the underlying case with hardly any force required. The case only needed a light sanding to make it like new. The two end caps also were quite easy to remove and they were left in a reusable condition.
All of this is good news. With a proper fixture to hold the sticks, I could separate the end caps and cells on a whole stick with everything ready to become part of another subpack.
No new parts required.
Time to get on the welding machine.