If the song itself changes BPM, as long as your timing sections are lined up with when the music changes, it will all by sync'd fine (assuming your bpms are correct). If the song doesn't change speed, and you're using this for effect, don't do that. It won't be sync'd properly.
Think of it like this: bpm is literally a measure of how many beats fit into 1 minute of music. A beat in 190 bpm lasts 315.8 ms, but a beat in 210 lasts only 285.7 ms. This means that if the music itself does not change bpm, every beat in the 210 bpm section is only around 90% (this is not exact) as long as each beat in 190 bpm, so every beat in 210 bpm is around 10% earlier than the last one. After one measure (4 beats) you will be early by 120.4 ms, and every beat at 210 bpm adds to this difference.
Technically speaking you can make bpm changes where everything lines up exactly the way it would have without the change, but I'm not going to go into that here because there's no reason to do that in osu. Your bpm should only change if the song actually changes bpm. If you want any sort of speed effects to emphasize part of a song, use inherited sections (green lines) instead of timing sections.