I know that this is a terrible photo but it will help explain:
The gearing cannot be changed to lower one half and not the other because of the design that suzuki came up with. If you look at the set of gears that are not in the case you can see that there are two 'levels' on the bottom two shafts but on the final shaft (at the top of the pic) there is only one gear. This means that they all spin dependant on each other and therefore gearing has to be of equal overall size so it all sits correctly.
Probably a bad explanation but someone might fix up if there is error
