Good points Andy. If it was just one dodgy cell then reducing each series to 12 cells would still be positive for me as it could bring the potential max voltage of the original battery up to 50.4v from its current 48.6v so still a success.
I think I'll look out for a big enough metal box that would take the charger and battery for charging in case of run away problems.
Full body protection and a safety mask for breathing.
Work outside if possible.
Have a large container nearby, filled with water, and a hose ready for any mishaps, to throw the battery into.
Electric cars that catch fire, are sometimes dumped into huge tanks full of water and given 12 hours to cool down........do not be in a rush to pull the battery out.....
Best wishes for a problem free fix.
regards
Andy