It isn't possible to change model lines directly into detail lines or vice versa.

Until now, we have used this technique:
1. Make a view that just contains the relevant lines.
2. Save the view as a DWG.
3. Import the DWG. If you need drafting lines, check 'Current View Only' (Uncheck for model lines)
4. Explode the DWG.

But now, we've found a new way:
1. Cut the lines to the clipboard.
2. If you need model lines, start a new sketch-based model element (e.g. a Floor); If you need drafting lines, start a new Filled Region.
3. Paste the clipboard contents into the sketch of the Floor or Filled Region.
4. Select the lines again, and copy to the clipboard.
5. Quit the Floor or Filled Region
6. Paste the lines back into the model.

It seems as though, when you copy lines out of a model-sketch, they become model lines, and when you copy them out of a filled-region-sketch, they become drafting lines.