In theory, you pull off the extensions, run the reamer and reattach the extension. You're not shortening anything, headspace stays "almost" the same as a regular 223. Here's a reference on the change.
http://www.accurateshooter.com/cartridge-guides/223rem/

Taken from the link..."This is because the overall geometry doesn’t change very dramatically other than the shoulder angle. You’re not blowing the shoulder forward into the neck."
Also, you increase the case volume around 7-8%, you get appx 100-140 fps more......All the more for a faster shooter, also looking at doing a switch barrel with my 223 savage.

I have an email into bert, i think he might kick my ass over this.