Just a comment about using functions like trim and extend. Please experiment for yourself and check the differences between the results that you get using enlarge sheet, or isometric trim as opposed to trim and extend or change edge. What each does to the shape of an extended piece of freeform surface differs, sometimes quite a bit. Each has it's relative merits and I'd be glad to hear of improvements made in later upcoming versions of UG.
If you have two or three slabs of freeform surface side by side and meeting at the edges with some degree of continuity it does not always follow that you can readily extend the free edges while maintaining that edge to edge condition between the slabs. Trim and extend makes an attempt that works on some surfaces better than others. You need to check it carefully.
This probably doesn't apply to all uses of UG and may not have been the originator's intent. I do however pick up a few models on occasion that have been worked on by users who find trim and extend all too easy and don't comprehend why I'm unhappy with their work when I find nasty face to face intersections or self intersecting geometry.
Regards
Hudson