A commonly-held belief is that if an automatic transmission has not been regularly serviced and is high mileage then changing the transmission fluid can cause it to slip or fail, but this is not quite the whole story. Regular oil and filter changes for an automatic gearbox is good practice, just as it is with engine oil, as fresh oil offers better lubrication and therefore more protection than old oil. A neglected transmission will experience greater wear of its clutch plates, and the clutch material that is worn off ends up suspended in the fluid, and this can offer enough friction that the transmission continues to operate with little or no slippage. Changing the entirety of the fluid within the gearbox can therefore result in the gearbox beginning to slip, as the suspended particles are no longer present.

We would expect the transmission fluid to be black if it has not been changed since new but the fact that it does not smell burnt is a good sign, and if you can check for any particles this also might indicate that the transmission is still healthy. Although 122,000 miles is further than ideal on the original fluid we would not expect the gearbox to be on its last legs, so would recommend changing the fluid - if the gearbox is in good health this will help it last longer, and as a safe option you can drain and replace the fluid rather than completely flushing the gearbox, so that some of the original fluid remains and will avoid dislodging any debris that has accumulated.