[robski]

Yes coverting to 8 bit and back to 16 bits does affect the image. It adds a very small amount of digital noise. In 99.99999% of cases I expect you would not notice it as you don't notice the digital noise when listening to music on a CD compared to the analogue version.

I made a simple image using the gradient tool in PS. On the original version you could see very light bands as the level changed. After the conversion to 8bit and back again the bands disappeared and a random speckling of noise was seen. I then amplified the effect by increasing the contrast on both versions to show the difference in the levels histogram. The upper window is the original and the lower is the converted. In the converted you can see a number of levels are missing. This misplacement of data produces the noise effect.

Ducan is on the right track with his reply.

Sorry for the techno babble but that is the nature of the question.