can the kick down on the gearbox be adjusted

My voyager has done 120000Km. As it gets older it is kicking own into a lower gear much more easily. when in spedocruise mode it will stay in 3rd gear for extended periods of time before it goes down to fourth. It also kicks into second to easily at times and the revs just climb dramatically. My mercedes could have the kick down adjusted. surely the voyager box must have the same option. How do you do the adjustment?. My local south African garage tells me it can't be done. I don'y believe them!