I googled this, but did not find any useful information. The LJ FAQ (mentioned below) only says that some clients can do it, but doesn't say which. Any suggestions?
FAQ Question #120
How do I make all my journal entries Friends-Only, Private, or Public?http://www.livejournal.com/support/faqbrowse.bml?faqid=120
Certain downloadable clients include the ability to edit journal entries, which can speed up the process of converting your previous entries to the desired security level by avoiding the wait for pages to load. For information on your preferred client, consult its documentation or menu options.