Our first question when a client tells us they're going to develop a new site is whether the page hierarchy and link structure is going to change. Most of the time the answer is yes… and that's when the fun starts. If you're an established company who has had a site for a while, migrating to a new CMS and design may be a great move… but not redirecting existing traffic is akin to SEO suicide.
Traffic is getting to your site from search results… but you just led them to a 404 page. Traffic is getting to your site from distributed links in social media… but you just led them to a 404 page. Social mention counts per URL now report 0 because social count apps like Facebook likes, Twitter tweets, LinkedIn shares, and others save the data based on the URL… which you just changed. You may not even realize how many people are going directly to 404 pages because many sites don't report that data to your analytics.
Worst of all, the accumulated relevant keyword authority you built per page through backlinks is now tossed out the window. Google gives you a couple days to fix it… but when they don't see any changes, they drop you like a hot potato. It's not all bad, though. You can recover. The image above is an actual client of ours that lost over 50% of all their organic search traffic, software demos, and ultimately new business. We supplied them with a SEO migration plan for the links but it was overlooked with the new site release as the highest priority.
That priority changed.
The company entered thousands of redirects into their server. After a couple of weeks, Google took note and returned them back to where they were. It wasn't without a lot of panic and sleepless nights by the team, though. The moral of the story here is that building out a new site with new link structures may be a fantastic strategy (SEO guys will sometimes argue that to the death) because of the increased conversions you can experience. But, but, but… be sure to 301 redirect all your links.
You'll still lose your social counts. We're experimenting with some ways to even stop that from happening where we keep a link structure for older content and then update the structure for new content. It's going to be fun!