Most of these 'upgrades' are gear reductions to acheive scale speed.

A very few go so far as to add ball-raced bearings on all the shafts, replace the brass gears with stainless steel gears and add enclosures to seal it from dust and dirt.

Some 'upgrades' are done because of use of metal tracks. The metal tracks increase tension and stress on the gearboxes and idler such that twisting and bending occur, so reinforcement is needed.

The cast metal idler broke off at the axel on one of my Tiger's after a year of running metal track!



Cheers
Eric Scott