That's actually not what you think. It doesn't actually start you over, it's just that any checkpoints that you have will be overwritten. Once you unlock a chapter, is always open for you to go back too later on in the quest select menu.
You can get back to normal mode act 1 even if your on inferno if you want.
It just warns you that any checkpoints you may have passed will be over written. You won't lose progress as you can alwas go back to where you left off at in the story.