You can adjust them via the Shortcuts.txt file, or by running the Keys.swf application.
By default, Previous seems to be assigned to the numeral 4 on the numpad, and Next seems to be numeral 5 on the numpad.
The next/prev feature is less useful with the Loader, because you end up with many "character" slots occupied by mod entries (such as ToggleHim). The next/prev logic doesn't understand the difference between these entries and actual characters, so it ends up cycling through them during gameplay. The result will probably be disruptive or confusing
(blonde ... brunette ... redhead ... wait a minute the male character just vanished wtf???), so it's understandable that the next/prev hotkeys were reassigned to less convenient keys.
@ModGuy probably assumed that nobody would bother using the next/prev feature anymore.