What do you mean by editing? Do you want to make changes to any of your objects (Alignments, profile, corridor, etc information) or just edits to the line work?
If you want to make changes to your objects, then you can export that information as an XML and the import it in a different drawing using your 2006 version.
If you want to make any changes to your line work only, then you can you export the 2007 file to a plain CAD 2004 format.
Cheers!
AG
C3D 2007, SP3
Phx, AZ