It happens every day. For whatever reason, eventually there will come a day when you decide that you need to change your website in a very big way. That could be part of a new software platform migration or part of a change from one website company to the other. Whatever the reason, if you rely on your website to generate business every day then that switch will cause a minimal of 3 months of website turmoil.
There's no way around that turmoil because users will be confused with the new interface as they try to figure out where you've "hidden things."
Of course, 3 months is an optimistic number assuming you take the time to fully examine how your website appears to the outside world before making the switch. You need to account for every existing page and product. People have been sharing your content and linking to you for years. Google and Bing have all your pages and content spidered and indexed, and now you're going to blow all those years of credibility out of the internet ocean.
Sounds bad doesn't it?
Before you do a major migration to another software platform or website you need to monitor the pages reports inside Google Webmaster Tools to see what pages are being linked to. You then need to pay for an account with SEOmoz and/or MajesticSEO and get your hands on historical backlink reports.
Using these 3 reporting tools, you should be able to build an almost complete list of all the pages that matter most to your business. This then becomes the master list from which you will need to build a 301 redirect library that will direct all visitors from the old website locations to the new website locations.
Need to know what a 301 redirect is? I have a few previous nuggets about it here and here.
Building this 301 library isn't something you should attempt overnight. Google Webmaster Tools doesn't provide historical information, so you will need active monitoring of your account for a few months before you fully see what happening out there. SEOMoz and MajesticSEO will give you some decent historical information as soon as you pay for it.
While you're working on the 301 project you should also start some serious user interface tracking on your website. This will allow you to find out the best ideas for creating a new website.
There's a serious amount of pre-planning before you actually move your website. You should plan for thinner bank account and for your competition to steal your customers away if you haven't taken any of the above steps prior to a website move.