After months and months of testing and planning, I finally migrated our forum tonymacx86.com from vBulletin 4.x to XenForo. More than a million users and threads in the 6 figures. It was a shockingly smooth move; I really anticipated running into... something. No matter how much testing you do, there's always something you missed or something you couldn't check on a test site. The only hiccups were benign template changes or some disk space limitations that altered how I wanted to do that move. One day later and our users are enjoying increased performance, amazing features, and a forum experience that's easily a 100-fold improvement. We haven't had any negative impacts with Google either, since I planned ahead with .htaccess changes. So, Bravo!