I just tried it. Requires some patience. There is zero extra room in the enclosure, so keep your solder blobs small, and do test fits as you work. Also take pictures as you work.
Tools: Soldering iron, flux core solder, T5 and T6 Torx, electricians tape.
Batteries: Panasonic 18650B (roughly 30% more capacity than OE battery) $15/set on Amazon
Remove the lens assembly. T5 Torx
Snap off the plastic top plate bezel
Remove the eight T6 perimeter/seal screws. Remove top.
Unsolder black and red battery/supply wires from PCB.
Remove old battery set, and all of the cardboard insulation.
FIRE RISK: shorted batteries can be a source of ignition. I had a small fire on my bench tonight.
Remove charging PCB (its on the end of the battery) sharpie mark the positive and negative.
Harvest conductive straping from old battery - you will reuse all of it - pry gently at welds with razor, clean with solvent and abrasive. The straps attached to the PCB may need to be cut if you can't pop the weld.
Wet solder small area on all battery ends. Overheating batteries can ruin them!
Wet solder layer on the corresponding area on the conductive straps
Using pre-wetted areas, solder conductive straps, to the battery ends.
Insulate areas at risk of shorting using electricians tape.
Solder the positive and negative strap ends to PCB.
Tape over component side of PCB
Apply electricians tape to all exposed conductors, but not too thick!
Reconnect red and black charge wires. To pcb in housing
Test1: Light operation
Test 2: Partially drain battery (charge progress bars = NOT full) then charge, observe increase in bars.
Close unit. Enjoy.