Snip away. All the tops can be replanted and will continue to grow. All the bottoms will sprout new stems, usually 2 at the node below the cut. This is how you get the bushy growth you see in a lot of scapes.
This is typically what I do with my ludwigia repens, just cut off the new growth and shove it in the substrate. The left over stem resprouts quickly and often times, two at a time. I typically just pull the old stem out and only use the new just because I prefer that look.The Ludwigia species may not like over crowding. In that case, pull the plants and replant the tops only.