This is the greatest tool ever. Posted originally in lj_nifty, someone has written a tool that will go back and change the permissions of all your public entries to friends only for you. Here is a link to the post. I hope this helps other people like me who wanted their journals to be friends-only. You need java installed to run it, which you can get from www.java.com.

*EDIT*
The original LJ-nifty post is no longer available, but here's a link to the hidejournal website instead:
http://james.is.never.wrong.nu/code/hidejournal/ There is also a hidejournal with more information.