No, do not make an NiMh upgrade. NiMh is heavy.
If you want to go forward use Lipo. Lipo is actual the most power in the smallest volume.
So you can use more capacity than NiMh and have more runtime and the Lipo can give you're ESC more power than an NiMh.
Charger:
Maybe check for Multiplex charger which have BID.
The sense of BID is that the values of the battery is stored in a small chip which have to connected to the charger.
The charger knows now what kind of battery and how many amps to charge. Additional to that you can see the charging history of charge times and how many amps where charged. The BID chip needs to stay on its own battery.
For example: I have different sets of Lipo´s for the Nero. On each pack is one BID chip.
When I want to charge them, I have just to connect the cables (Main cables, Balancer and BID-chip).
After connecting them I just had to push the start button. Never think about to make an mistake.
Also it is an girl-proof solution!

I have the Multiplex D7 but with BID there are also smaller ones.
Just an suggestion.