Forum Moderators: mack
May God have mercy on your soul.
Make sure the company brass know to expect a bumpy ride in the search engines for a while, if the search engines have been a signficant source of traffic for you.
Have a plan for salvaging your existing link/bookmark equity. Identify everyone you can who is linking to any page on the existing site. When the new site is ready, contact them to request that they update their links. Some will, some won't, but do what you can.
Give some thought to what the user experience will be if someone happens to follow an old link or bookmark. Figure out how you'll guide them to the equivalent content on the new site if possible. Try to let users know what's happening rather than just redirecting them without explanation.
Offline: Have someone in the company list places where the existing URL appears in offline promotions, and make sure there's a plan for updating those too. There might even be places where you'd want to publish the new URL "early" and redirect type-ins to the old site if the new site wasn't fully ready yet. Phone book deadlines are one reason you might want to do that.