I'd be interested to hear from those with early As who will know if I'm wrong but all the ones I know of have a choice of gearing for sidecars, this only involves changing an external gear (engine or gearbox) to lower the overall gearing, gearbox remains the same.
There are different gear ranges ( scramble, road, racing ) that are fitted to the same gearbox, the box is then (if factory fitted) stamped with the ratio code and there is also a RTT2 box that is internally different in having needle bearings in place of bush's.
I think I'm safe in saying though that for general sidecar work only an external gear needs to be changed