"Halve BPM" will halve the number of beats until the next tempo change, except that if there were already an odd number of beats (ie. 3), the last beat before the tempo change is anchored and left as-is.
Could an option be added to shift tempo changes to the nearest position that creates an even number of beats? I realize this throws the whole midi (and in turn the notes) slightly out of sync, but it's a logical first step to properly fixing a double-bpm chart that was generated with tempo changes on odd counts, rather than end up with the odd-man-out beats twice the bpm of the pairs before. As it is I'm doing this by hand prior to using the halve bpm function. Essentially, I'm talking about a break-sync-on-the-way-to-fixing-sync function, and I realize that may seem a bit illogical.... if this were to prove plausible it should definitely include an "are you sure" window reminding you that the resulting midi will need to be re-synced.