Do nothing, and the receiving device will truncate, or chop off the bits that it can not handle. Only if your audio device or software has an option to add dither can the decision to choose dither be made. You may be able to add dither after the fact, alleviating the need to make a permanent irreversible decision on the spot. Either way, the 8 dits are lost. Dither is a way of compensating for the truncation's inevitable increase in audible quantization error before the truncation occurs.
Bit depth represents dynamic range in digital recordings.
A recording that is done in 24 bits will have a greater dynamic range than a recording done in 16 bits. Dynamic range is the difference, from softest to loudest, that a recording device is capable of. Dynamic range specs will include system noise, so that signal to noise ratio will always be less than dynamic range. Analog systems have some inherent system noise. Digital systems introduce no system noise, so where the noise factor enters in is with something we call quantization error.