A snippet for bulk converting Facebook privacy modes on all currently visible posts in Activity Log.

ActLogBulkPrivacyMod.js

// Mode to find, and mode to switch to, as shown in the privacy dropdown.
var from = "Public", to = "Friends";
// Gather all posts on the old mode.
var posts = $$('a[aria-label="' + from + '"]');
// Assuming running from the console, copy the $$ selector method.
window.$$_ = $$;
// Facebook overrides setTimeout, so need to obtain one from elsewhere.
// Don't destroy the frame, otherwise it'll stop working.
var f = document.createElement("iframe");
f.style.display = "none";
document.documentElement.appendChild(f);
window.setTimeout_ = f.contentWindow.setTimeout;
// First pass: preload all the menus (will hang the page).
for (var i in posts) {
    if (typeof posts[i].click === "function") posts[i].click();
}
// Second pass: open each menu in turn, select the new privacy option.
function proc(i) {
    posts[i].click();
    // Wait a moment... (using the stolen timeout method)
    setTimeout_(function() {
        // Now find and click the desired mode.
        $$_('li[data-label="' + to + '"] a', posts[i].nextSibling)[0].click();
        setTimeout(function() {
            proc(i + 1);
        }, 100);
    }, (i === 0 ? 500 : 100));
}
proc(0);