We do get a fair number of those requests as well ("I have FO journal, but I want to go back"), and digging the 'public' parts of that FAQ out is fairly difficult. The wording is all geared for FO or Private, and it doesn't really need to be. For setnewpost minsecurity especially, the 'public' reference is currently one line at the end of that section.
So, the following text is a re-done FAQ that is phrased to be more generic, appropriate to any security level they are asking about. It is a first suggestion--feel free to hack it apart further and so on.
Altered FAQ text
I tried to generally highlight changed areas in italics. The one bolded phrase near the end is because I am not sure about that--do they need to be logged as the maintainer to change the minsecurity for a community? Or even if they don't "need" to be, do we want to prompt them to do so anyway?
How do I make all my journal entries Friends-Only, Private or Public?
To make all of your past entries Friends-Only, Private or Public, you must edit each entry individually (http://www.livejournal.com/support/faqbrowse.bml?faqid=2) to change the security level on each one. LiveJournal does not provide a way to change the security level on all your past journal entries with one command. See http://www.livejournal.com/support/faqbrowse.bml?faqid=24 for details about the various security settings.
Many clients available for download include the ability to edit journal entries. This may speed up the process of converting your previous entries to the desired security level. For more information on your preferred client, consult its documentation or its menu options. See http://www.livejournal.com/support/faqbrowse.bml?faqid=158 for information on downloading a client.
LiveJournal does provide the means to set a minimum security level for all your future posts. For example, setting this option to Friends-Only or Private will prevent you from slipping up and posting a public entry that you meant to post at a protected level. You will still be able to post an entry with a lower security setting by posting it with the minimum security you have set, then editing it to change the security setting.
To set the minimum security level of all future entries in your journal, log in (http://www.livejournal.com/login.bml) and go to the Admin Console (http://www.livejournal.com/admin/console) and execute the following command:
set newpost_minsecurity <level>
Do not enter the < or > symbols; these are only for reference purposes. To set the minimum security level to Friends-Only, replace <level> with "friends". To set the minimum security level to Private, replace <level> with "private". To set the minimum security level to Public, replace <level> with "public".
If you maintain a community, you can set the minimum security level to either Friends-Only or Public. You cannot set this option to Private for communities.
To set the minimum security level for your community, log in as the maintainer and access the Admin Console as described above, then execute the following command:
set for <community> newpost_minsecurity <level>
Do not enter the < or > symbols; these are only for reference purposes. Replace <community> with the username of the community. To set the minimum security level to Friends-Only, replace <level> with "friends". To set the minimum security level to Public, replace <level> with "public".