Ver. 8 / How to: change tempo, change instruments?

[Gatto]

Ok, that's fine but known... This is an excerpt from Revision History...
Improvements...
- Change tempo during playback.
- Change instrument sounds during playback.
So, please, how can I make these changes?
For example, my goal could be to have some measures with piano, while bridge or other measures with guitar. And of course, save this change to the song and play that way every time I reload the song. I think it is impossible for everyone playing his/her instrument and change tempo and/or playback instrument at the same time. These changes must be part of the backing tracks...

Grazie.