Hi Lewis, if you want to keep 100% exactly the same shape you can untrim it and then retrim it. To untrim you need to use Edit > Separate on it first so it's individual surfaces, then select all edges (either select one edge on each surface and use Ctrl+A to select all edges or use the Scene Browser > Types > Edges click the selection dot area on the right) and hit Delete.

And before doing that select all the edges you'll need for retrimming, use Ctrl+Drag or Ctrl+C/Ctrl+V to duplicate them and then squish them flat in a side view using "Flat snap" (http://moi3d.com/forum/index.php?webtag=MOI&msg=3378.4), then you'll use those as the cutting objects in Trim.

If you want to construct a new shape, Sweep is good for building a potato chip like curved shape from 2 crossing curves, you can select one as the profile and one as the rail even with the profile in the middle of the rail. That will build a 4 cornered sheet, then Trim with a circle or oval shape 2D curve.

- Michael