modify tempo afterward?

You could use Reaper's digital varispeed. I believe that should work. It's what I do when I want to alter a master files tempo. Even better you can tell Reaper not to alter the key.

Ben

you can do this in CbB!!!!
 
highlight all your clips and right-click and clip lock "position" all clips. Make sure that each track time base int the inspector is set to "absolute" and not "musical".
 
now you can change tempo without any issues.  you can use "Set Measure/Beat at Now" to get the click to match your audio if you can find where the beats are in your audio.
 

Since you apparently recorded without a click (?), I'm assuming that 87bpm figure is only approximate, in which case you should do this to get a more precise sync of the timeline to the audio:
 
- Assuming no partial 'pick-up' measure, trim the earliest clip(s) to the first downbeat transient, and slide everything back to start at 1:01:000.
 
- If there is a pickup measure slide all tracks so the first downbeat transient is at 2:01:000, snap the Now time to 2:01, hit Shift+M (Set Measure/Beat At Now), enter Measure 2, Beat 1 and OK.
 
- Play the track with the metronome off, and count measures/beats in your head out past the start of the latest clip in the project, and stop playback on a downbeat with Spacebar.
 
- Shift+M, enter that Measure and Beat, and OK.
 
SONAR will adjust the initial tempo (or the one you set at 2:01) to make that measure/beat in the timeline fall on the absolute Now time where you stopped playback. The absolute playback timing and sync of tracks will not be affected.
 
- If you had to set 2:01, edit the tempo at 1:01 to match it.
 
- If you find the timing of the audio is drifting around that average tempo (likely), you can set the Now time to other measure/beat transients before or after the last Set point, and Set those additional points as needed to really to lock in the timeline everywhere.

A valid point, in general, but it sounds like this is more than just one track, maybe including instruments the OP doesn't play, and/or performers that not available, and who might not know the song anymore years (?) after it was recorded.

You should use the method I posted above. I do it all the time.  I have a client that will record multiple tracks of a voice over for a info /promo videos and I have to send samples of different music beds.  The music beds will vary in tempo and if I can get the click track to match the music bed it becomes very easy to edit the music to match the picture.  I can move things by measures and beats instead of constantly finding edit points in the music clip.
 
So I like I said...I clip lock the audio clips into absolute position and then I change the tempo at will to match the music bed.  At times I will add a crazy time signature in just one measure to get beat 1 to line up (like 19/32). Since I do it so often it only takes me a couple of minutes to do this even if the music track is changing tempos (tempi). 
 
Send me the Cake file and in 5 minutes I'll have the click match the music for you.  If the music was recorded with out a click I'll spend another 5 minutes and approximate it for you.  Ill send you back the Cake file.
 
PM me if you'd like

brundlefly

Jimbo 88
I clip lock the audio clips into absolute position and then I change the tempo at will to match the music bed.

The advantage of Set Measure/Beat At Now is that it calculates and inserts the exact tempo(s) for you, instantly. No guessing, and the result is dead-on, not approximate.

 
Yes, exactly.  That is what I do most of the time.  There are multiple ways to do this, but if I know for sure that the tempo is say...87bpm... and it needs to start a couple of seconds into the file, I might use a off beat measure instead of having different tempos.  You have options and many different ways to skin a cat. 

bitman
Don't change the tempo.
 
You have melodyne, good. take a good rhythm track like a guitar and drag that track to the timeline.
Presto chango the tempo map now follows that rhythm track. Sync your midi to that.

Oh Yea!  There is that one too...but still have to lock the clips or drag all the clips to the front.
 
Good one Bitman!

GregGraves
 
 You can do anything all over from scratch, and it is the rare situation that the redo is not better than the original. 

I wish I could believe that. but I cant. You see its not just rare, its indeed a fact that each take has its own personality and feeling.
Capturing lightning in a bottle isn't just a catch phrase. Ive spent years studying, trivializing the fundamentals of replication. Its not as simple as one would think.
Its not just BPM, its several bpm, drifting at different times. Its not just plucking strings but attack of those strings.
 
For most intended purposes, yes, one can replicate a track rather easily.
 
But the percentage of capturing the same feeling... of said track, that's not as easy as hitting the record button and trying to "Redo" the track.