Here is one script ("Revert Curve" in the contextual menu).
Actually, there is no simple method, because changing the order of segments impacts the whole model structure for Sketchup.
The attached script works fine on isolated curves.
On curves which are linked to faces and other edges or curve, the scripts has to erase the edges and reconstruct the faces. So you'll loose the material and other properties of these faces.
EDIT 01 Feb 10: Attachment removed. See this post to get the plugin
http://forums.sketchucation.com/viewtopic.php?f=323&t=25710#p221170
Fredo
PS:
The contextual menu item appears on a selection which contains one or more curves. Elements in the selection which are not curves are simply ignored.
You do NOT need LibFredo6.