Convert positive elevations to negative elevations

Create a volume surface by comparing your existing surface with flat surface created from your surface outline (feature line) and then pasting that invert surface into new surface helped.

1. Create a feature line at Elevation 0 that completely encompasses your surface.  
2. Use the Feature line to create a surface.  
3. Create a Tin Volume Surface with the zero based surface as the Base and your surface as the Comparison surface.
4. The resulting volume surface will be the invert of your surface.
5. Then paste the volume surface into a new surface it can be used normally e.g for profiles and sections.