I usually start by Save As (project name - NEW TEMPO) to protect what I've done to that point, then edit dron that file.
To use audio snap for this:
Bounce all audio to single clips that start at zero (begining of the timeline)
Highlight all audio clips.
Click View, then Audio Snap palette
Click the option Clip Follows Project to turn it on (check the down arrow to the right of Clip follows project and make sure it's set to auto stretch)
The clip should now have red double arrows towards the front of the clips to indicate they will follow the tempos
Now you can change tempo(s) freely and the tracks will follow. They will be using Groove Clips algorhythm for playback so it won't sound as good until you decide on a tempo and then highlight the clips, right click and bounce them down.