For a long time I’ve wanted a way to take my old ‘low security’ encryption keys, and make them subkeys of my current ‘high security’ main key so that I don’t lose it or forget my passwords. Atom Smasher has found a way to do this in gpg.
Really, this should be a drag-and-drop operation, followed by entering a pair of passphrases. But at least this complex mess seems to work.