There is no default time for custom events. You can set the time for anything that you like when you create the event. The settings for Birthdays and All Day Events only give you the choices that you see in this screenshot. So to answer your question, no you cannot change from 9:00 AM for these events unless you choose None and select your own time
If you want to change the default time to None the path is Settings>Mail, contacts, calendars>Calendars>Default Alert Times>Birthdays or All Day Events.
