If I redirect the OldDomain domain to the NewDomain domain will I lose any traffic?
The short answer is Yes you will lose traffic.
For the details, please see our [url=http://www.highrankings.com/forum/index.php?showtopic=12535]aging delay thread[/url]
In a nutshell, your new domain is going to be under Google's aging delay restriction for many months. Even if that were not the case, setting up a (normal) 301 Moved Permanently redirect from the old domain to the new one is not going to immediately transfer any link popularity. So even the other engines are going to take several months to catch up with the changes.
About the only chance you have of maintaining some rankings is to set up a 302 redirect from the old domain to the new one. Doing this can cause the engines to continue showing the old domain in their SERPs, however real traffic will still be redirected to the new domain. Work on getting links, etc to the new site as you normally would. Then months down the road when the new domain has emerged from its aging delay you can switch it to a 301 redirect.
This of course means that you'll need to set up hosting for both sites in order to control the redirect status code that is being delivered.
Note also that I said above that a 302 may
work. Google have stated in the past that it's their policy to reset the link popularity, etc of a domain when it changes hands, presumably in order to keep the turn-and-burn artists from simply snapping up existing domains to overcome their aging delay. So even if you do everything right they may still drop the old domain because they've detected the change of ownership via the WhoIs records or other changes you need to make.