Except for the bell housing, the transmission is also the same as in the Citroen C1, Peugout 107, Toyota Aygo. To change the fifth gear, you would need to take one apart. They do not have an exterior cover that allows easy changing fifth gear without taking the gearbox out, like most FIAT for example.
As long as no bearings or shims are changed, it should be relativly easily possible if you stick to the FSM.
BTW, AISIN belongs to Toyota.