You can actually upgrade

Unlike the rest of Adobe's imaging applications, which do a terrible job of helping you migrate to a new version, you can actually upgrade Lightroom. The installation won't overwrite the older application, but it provides an automated way to upgrade your catalog, including catalogs created in Photoshop Elements. While LR3 doesn't have the face-recognition capabilities in Elements, it will import all the tags that already exist for recognized faces. Still missing, however, is the ability to import a copy of the old catalog and continue from there.