This is something that I have to do this weekend.
If you're not planning on expanding the stand of Ammania, then just cut off the top 8" of the stem, pull out the rooted base and replant the tops. This will keep the look of your existing plants, but make the stand shorter.
If you have room to expand, then cut the top 2/3 of the stems and plant these tops in front of the rooted bottom portions. This will hide the cut stems and allow them to grow side shoots which will fill in behind the tops.