Thanks Cameron.
On further inspection, the part of the model that I am interested is actually NOT georeferenced – only a few of the cross sections somewhere else in the model. Nothing shows up in RAS mapper.
So I think I need to do the following:
Redraw the stream reach I want to edit in GeoRAS (the length of the reach should match), draw cross sections at locations of the previous manual cross sections, cut the cross sections with GeoRAS, then update the manual cross sections with the new elevations, then create new interopolated cross sections.
Is there a better way of doing this?