It has 4 dedicated sequencers. They're locked to whatever tempo the metronome is configured for. You can mix durations pretty fluidly with the endless sequencer (8th, 16th notes/rests), but you definitely cannot change tempo in a given beat--at least not without jumping through a bunch of hoops bouncing and re-recording tracks, etc. There's a basic grid type sequencer as well, but that doesn't meet your criteria.

This was like five years ago that I had one, but as far as I remember, OP-1 isn't grid-based (though maybe some of the instruments are?). It's basically like a four track tape mixer. You can loop bar-by-bar, and then copy from bar to bar. And you can stretch time in the same way that you would with a reel-to-reel tape deck. But as far as I remember it won't transpose / warp the sounds along with the change in BPM.