PostConscious is a bulk security editor for LiveJournal and clone sites. It is written in perl, has been tested on Linux, FreeBSD and MacOS X but should work on Windows with ActivePerl. It requires modules XMLRPC::Lite and Date::Manip (can be installed from CPAN or through ActivePerl). It can be downloaded from here.
PostConscious is careful with security editing, and can be used to turn a sequence of posts from one security type to another, namely from public to private, friends only or custom grouped without resetting permissions on custom grouped or private postings (although if you want it to affect these posts, you can tell it to). PostConscious can also modify security in different permutations, for example private to friends only, or even turn your whole journal public (if you so desire).
I hope it's fairly easy to use. Please let me know if you find any bugs!