Well, that trick worked, mostly. I copied the nice, tight drum track from the 120bpm project into the original, 135bpm project, and started re-recording the bass, locking in to the new timing.
Here's a teaser, from the end of the guitar solo where everything is getting quite excitable:
Just listening to this, I can hear some places where the drum velocity levels should be tweaked a bit. The toms are a bit loud and brash, and could do with a little less reverb.
Still... it's much better. Now to adjust all the other instruments to fit!