Is there a way to flush out old preview files? If you change one slight thing in the video it has to re-render the preview, and from what I can tell by my massive folder is that it keeps ALL the preview files. Its eating up many gigs of my drive for an 18 minute effect heavy short film. Is there a way to can the old previews?

Yes, easily. Sequence > Delete Render Files.

i do not wish to clear them ALL, just the unused ones. Is that what that will do? Or will that delete all of them? Just want to be clear and sure.

I think it blows out the whole batch. The names are pretty cryptic and I'm not sure there's a way to delete just specific ones.