In addition to this:
I think once a clip has been recorded it should show its BPM in the clip details. Something like: "original: x.xx BPM". Later, if you change the global clock you should be able to use this clip's original BPM:
  • a button "use this clip BPM for the global clock" in the clip details
  • action (for button widgets etc.)
Oh and really cool would be, if you could have a smooth transition when switching the tempo (speed up, speed down).