Whenever you’re working within Lightroom, you’re working in a catalog. A Lightroom catalog contains all the information about the image files you’ve imported, as well as any adjustments you’ve made or metadata you’ve added to them. The catalog is a file residing on your hard disk; see figure below.
A catalog file is specific to the version of Lightroom that created it. For example, a Lightroom 2 catalog is different than a Lightroom 3 catalog. When you upgrade Lightroom between major versions (such as from v2 to v3) — and sometimes between “point versions” (such as from 3.0 to 3.2) — you also need to upgrade the catalog.
When you launch the new version of the program, Lightroom looks for the default (or most recent) catalog used, and if it finds an older version, you will be prompted to upgrade the catalog. You must allow this upgrade to be successfully performed before you can access your old data with the new program!
During a catalog upgrade, the Lightroom installer copies your old catalog to a new file and then performs the upgrade to the copied catalog. As a result, you end up with two catalogs – one from the old version and one for the new version.
After you’ve performed a catalog upgrade and confirmed the integrity of all the data it contains, it’s imperative that you remove the old catalog(s) from your system. This will prevent accidentally opening the old catalog when you didn’t mean to.
If you have multiple catalogs from the old version of Lightroom, all of them will need to be upgraded to support the newer version, and then all the old catalogs should be deleted. (If this makes you nervous, back them up first.)
Also, at this point you should completely uninstall the old version of Lightroom.
If you ever launch Lightroom and are unexpectedly prompted to upgrade the catalog (and you haven’t just done a program upgrade)… STOP! If you’ve already upgraded this catalog, don’t do it again – you will just end up with more copies of the same catalog. One of the worst things you can do in Lightroom is work in multiple catalogs without knowing it!
You can always confirm the catalog that’s open by using the Catalog Settings command. On Mac, it’s located under the Lightroom menu at the top left of the screen. On Windows, it’s under the Edit menu.