Just put any other type. Will be twice as big but who cares? Can be done in couple of hours
The charger and battery protection systems need to be matched with the battery cell technology, so unless that already supports a different cell chemistry (eg NiMh or NiCad) then no you can't just fit a different battery tech in a few hours. There are likely to be other issues as well. Is there an NiMH pack available with all the same connectors/signals in the right places?
from an earlier post...
They also said a battery fire could not happen in flight because the APU battery was only used on the ground.
So it's not charged in flight either? There have been cases of Li batteries combusting when not in use, sometimes weeks after the original damage occured but this wasn't a Li Manganese cell technology.