Apparently you can change tempo and the video will adjust its speed to match.

What happens when you change the key? Does the guitar or keyboard player change how they are playing or do they continue to play in the key it was recorded in?

I think if you change the key, you need to reproduce the entire video. My understanding is that the video changes to suit. I did try transposing a test from G to Eb. However, I did not test if every possible key is available.