Develop new site while old site still available

I have an existing site that uses Domain Access that I'm going to re-develop. The current site resides at http://brat.info (the primary domain).

When developing the new site, I'd like the current site to still be available to users. From what I've read in the Drupal forums and heard from friends, I can setup the new site at something like http://new.brat.info and work on that while users still access and use http://brat.info as normal.

My question is however, how do I setup Domain Access on http://new.brat.info so that it works while I'm developing it, but so that I don't have to change anything (or much) when I move it back to http://brat.info when it's complete?
Will this work at all? What would I give the domain names on http://new.brat.info (new.domain1.com or just domain1.com)?

Any assistance would be much appreciated!