To recap, iMovie '08 converts AVCHD to .mov Quicktime movies upon import (using Apple Intermediate Codec). There is no need to export the clips again in order to use them in iMovieHD. Just follow CMU's advice above: set up your new movie options in iMovieHD, open the iMovie '08 event folder, and drag and drop the clips. You'll be editing clips of precisely the same quality (because they're precisely the same files) that you'd be using should you choose to edit in iMovie '08.