I read it somewhere (www.usvisanews.com?). You can. But you have to stay outside of the US from the end of your 6 years utill one year from your filing the extension.
Another possibility is that you can get your new LC approved before your 6 year limit is reached if PERM is implemented soon.
In both cases, you will have to find a new job and start your LC right away. But the advertising takes up to 6 months.