How do I convert a 3D solid object to a 3D surface?

There might be an easier way, but one method would be to initiate the Offset Surface tool, set the distance to 0, then click in the graphics window (off the part) and hit ctrl+a, and it will select all of the faces. Then you can hide or delete the solid body.

Why does it need to be a surface body?

If you delete a face using "delete face" with option set to "delete" (not patch or fill), the result is a surface body with the one face missing.  If you copy or offset that face before the deletion, you can knit the results together.

If the result is to be exported to another format, then it probably doesn't matter if it is surface or solid.