Originally Posted by SplitNeo

if both servers are up during all the move there should be no downtime.
Also, the whole concept of propagation is kind of past now, with higher speeds and high processing power nowadays it's not every isp that has DNS caching servers, making everything faster.
So once you have made the change, it should come up almost instantly, as soon as you update the values, you just need to be sure that you don't have the site cached on your own machine.
To flush your DNS cache (using windows) you do open the command prompt and run: