... Replace all batteries, using NMH
rather than NiCad, as they have larger capacity...

This can't usually be done, because the end of charge algorithms
on the smart chargers won't won't be compatible with the different
cell technology. It's very possible a charger for NiCads can grossly
overcharge NiMh. You had better HOPE the circuitry has a timeout
value, otherwise you'll melt them down to a shapeless blob.
If nothing else, this will bring the life of your new NiMh's down.

You have it backwards. NiMH cells are usually pushed to a higher
end-of-charge voltage than nicads. Charging NiMH cells on a nicad charger
will, if anything, undercharge them.