You can also convert to scalar, and pipe back into a displacement shader which will than give you a displacement multiplier.
Though by what you're explaining, I'm not sure why not changing the PF/Heightfield displacement multipliers wouldn't work. You shouldn't need to edit the PF fractal scales, but the displacement amplitude.
If you mean changing the scale of the whole setup, without effecting PF settings, you can use the same method from line one, except adding a transform input shader before the displacement shader. See example files.