Lets say I build tons of links to my root domain and get my site ranking on the first page of Google and it's a simple blog. Once it is ranked if I put up a new site in it's place what happens?
It will hold that page rank until it updates again. People use to do that all the time. They would have their main domain direct at google so it looked like one of google's thousand domains. It would get a PR 9 after the update and then they would change it to their own site. Don't know if it still works though?
You would lose the placement, and everything would get re-indexed. All links that did work would be redirected or show up a broken to Google. Doing what you are asking about is not a Good idea when it comes to SEO.
This can be done for a short period of time by using a "NoIndex" meta tag on all pages, but Google will finally index the site and everything will change to reflect what's there.