Hi Samuel, yes wanting to move a vertex around is a very polygonal modeling type approach.

NURBS modeling tends to operate around a very different approach, much more focused on using cutting and boolean operations to build things.

In a case like you have there, "changing the top surface" will usually be done similar to what Pilou shows, by making some extended larger surface (by whatever means you wish, you can use lofting or control point editing like Pilou shows), and then using a boolean operation to slice your main object into 2 pieces with the "terrain" surface, leaving the trimmed terrain surface on the top.

That looks something like this - your base object:



Construct an extended terrain surface, using whatever surface construction method you want like loft through cross sections like here, or any other surface creation tool. The key thing is that the terrain surface will push through the base object, note that this is not trying to construct a surface that directly hugs the irregular outline of the top of the base object, it is an extended surface and you will rely on intersecting that surface with the base object to trim it off to the final outline, rather than trying to "sculpt" individual vertices around like you would in a polygon modeler:



Now select the original base object, and run Construct > Boolean > Difference and select the wavy terrain surface as the cutting object. That will divide the base object into 2 pieces like this:




Delete the piece you don't want and you will be left with the trimmed terrain surface on top:




This concept of initially building a larger extended piece that then gets cut to the final outline is a key thing for NURBS modeling and it's a very different strategy than the individual vertex movement type approach that is used with polygonal modeling.

You might want to check out some of these tips for people coming from a poly modeling background:
http://moi3d.com/forum/index.php?webtag=MOI&msg=4865.2

Hope this helps!

- Michael