As you have discovered, once your assignments are on the calendar, changing the beginning of the school year won't automatically shift everything, so you will have to go in and bump the assignments yourself.
What is happening when you bump? Is it not skipping the breaks? One thing you might try is bumping one lesson plan at a time to see if everything falls into place. There might be one or two assignments somewhere in the list that are throwing everything off.
If all else fails, you could also unassign everything quickly by removing lesson plans from your students' Learning Plans (this also removes all those assignments from your students' Calendars) and then reassigning them to start fresh. I only suggest this because you haven't actually started school yet and won't risk losing any completed work.