DGNEXPORT to remove line types:
1. Open the affected drawing file using AutoCAD LT.
2. Enter DGNEXPORT at the command line.
3. Select V8 DGN under 'Files of type' and click Save.
4. When the Export DGN Settings message box is displayed, verify that the correct seed file and units are selected.
5. Click OK.
6. After the DGN export is complete, open a new template and enter DGNIMPORT at the command line.
7. Select the newly created DGN file and click Open.
8. Verify that the Conversion unit is correct.
9. Click on Mapping setups and select new and make sure Mapping type is Import.
10. On the new Mapping setup, click on Modify.
11. Go to the Linetype tab, change all rows to ByLayer (right-click on the first row which has been set to ByLayer and the select Copy, then select all the rows and right-click > paste).
12. Click OK.