Hi,
so you've recorded tracks at 120bmp (for example), and are now switching to X bmp? I'm assuming the content is an audio track, and not midi or anything.
Unfortunately, the only option you have is time-stretching (plugin) your audio, which changes the pitch of the audio as well (depends on how much you're stretching it).
A program like Reason can adjust to tempo changes as all the synths, drum machines etc. are midi and run off the same 'tempo clock' in the sequencer.
It's best to find what tempo the song sounds best at, create a click, and record all audio and midi to that tempo. This doesn't mean that the whole song has to be one tempo...PT lets you edit your tempo map as you need to.
or
record the basic rhythm track as is and how you 'feel' it. Go back to where the start of the first measure is and 'Identify Beat' (cntrl-I in XP, Apple-I in OSX) as 1 | 1 | 0 | 0 . Do this for every beat in succession (ie. 2nd beat 2 | 1 | 0 | 0 and so on).
Once you've 'Beat-Mapped' your entire song, the click and grid mode will follow the map you just created. The bpms will change every bar no doubt, as you didn't record to a click.
One option from this, during playback, is you can look at the bmps change and find an average bmp to your song, create a click and record everything to that, or see where your song slows down, and adjust the click for this region only...etc. (Essentially Beat Detective)
Ran out of time...bands here...
-LIMiT
