You need to submit an expedited review request. From my personal experience I've generally pushed these out within 24 hours. Usually Apple does these as a one time exception, and is supposed to be used once in a great while. I've done 4 total in 2 years. Anyhow, since there is contact information on the website, if users download and contact you reply back that there are pending updates, and will be out shortly.
You could go through pausing delivery of app until ready, etc, etc. Availability can be changed and automated via dates. You can remove from countries available.
Here is link for expedited review. You'll need your App ID and Name, etc.
http://developer.apple.com/appstore/contact/appreviewteam/