I've just discovered I have built a complicated DE in a wrong form, then needed a tool to move it to where it belonged. The closest I could find was my FAQ #7978, but it worked only when you created a new form from scratch, not when you edited. Well, that was a start, and in the end the little ten-liner works for both cases: when you want to copy a DE from an old form into a new one, or when you want to copy it from an old one into the one you're currently editing.

On the flip side, you must save the edited form under a different name (or location), because the old one is still in use (by you), so it takes some file manipulation afterwards.