I'm working on a song at 80 BPM, and I changed it to 160 BPM by changing the song form. Did the Edit>Song Form>Expand. The bars expanded, the BPM changed... Except the markers where the substyles change did not move. So instead of maintaining verse/chorus continuity, it's just a scattering of substyle breaks.

It appears that expanding the bars works just fine if you work in 4 bar sections (or 8, 12, so on). However, if your section is 5 bars or 9 bars, then it doesn't recalculate where the breaks should move to.