RoboHelp (nor Flare) doesn't have or support change bars, so the FM Publish utilities won't have them either. You'll have to come up with some other method of indicating changes in the CHM. Possible candidates include Character tags (you can search & replace for Change bars using Character Formats) or Conditional text. In both cases having different background colours for different versions/releases would give similar results to change bars (bu you would then be able to track changes over multiple releases, whereas change bars have to be reset every time).
> change bars have to be reset every time
I feel a nuance coming on … in an earlier position, I produced goodness knows how many releases of some regulatory documentation; management were delighted that with judicious use of conditionals we could save all the release history in the same file(s). Version 1, new_v1 condition set to display with change bars and deleted_v1 with strikethrough; Version 2, new_v2 with change bars, new_v1 without, deleted_v1 hidden. So I was resetting the conditional text display, but for my own benefit.
A pity that .html-based ouput hasn't caught up with .pdf, though.