When I first started in FCPX I had the "Copy to library storage location" checked on import settings. This was all great and dandy until I realized my storage requirements are climbing higher and higher.
Is there a way to convert projects from containing full duplicated media files to having linked media files?
Not automagically. But you can simply remove them from the Library and then relink to the originals manually via the Finder. But beware- make sure the original clips all match and relink properly before deleting anything.