It looks like you have a fundamental misunderstanding about NURBS surfaces. The topology of a NURBS surface is a rectangle. That's what you had when you lofted your original surface.
When you trimmed away all or part of it's boundary, that new trim curve was added as a modifier to the surface definition. It does not update or change the underlying topology. It just visually ignores anything outside of the trim curve. All the original geometry is still there.
Based on your description, you will need to replace your trimmed surface with one that is not trimmed.
I would detach the trim curve, Explode it, and use the Section command to take slices through your existing surface. Choose curves appropriate to the surface creation command you wan to use. Perhaps Loft or NetworkSrf. Then use these harvested curves to make a new untrimmed surface and delete the original trimmed surface..