Convert PCM to Dolby without rebuilding?

I have several DVD titles that were made a while ago and have PCM audio , I would like to convert the audio to dolby to save room as the audio in PCM takes up more space than the actual movies.

I know the old way of doing this is converting to video files I can work with & rebuilding the DVD using new menues etc. The current menus are really well done and I would like to keep them. Is there an easier way? Is there a program that can keep the DVD as is (menu structure etc.) and convert only the audio to dolby?