Hi Danny - I think you already have the answer to your original question, that you can use Delete to "untrim" a surface (and recover the underlying surface) if you have all the edge curves for a trim boundary selected.

Sometimes for this kind of "low level" object repair procedure you may also want to copy several of the edge curves to the clipboard first before doing the untrim, so that you can paste back in some that you want to keep the same to use for the next re-trim.

There is a tutorial that I did back a while ago here: http://moi3d.com/forum/index.php?webtag=MOI&msg=446.17
which covers this kind of low level object repair techniques in some detail. That one was to repair a boolean that was done in a wrong location but not noticed until later on.

The basic procedure is to use Separate to break solids into individual surfaces, untrim surfaces, re-trim them, and use Join to glue things back together.

One thing to remember is to untrim you must select all the edges that make up the trim boundary - that means a full trimming loop not just one edge that is a portion of the trim loop.

- Michael