Per the user's guide, from the Library Module I: File > Upgrade Photoshop Elements Catalog
This brings up a dialog box that shows the most recently upgraded PSE catalog. Then, to quote the user's guide, "To choose a different PSE catalog, click the Elements drop-down menu."
There is no "Elements drop-down menu" within the dialog box to click on. The closest thing is an expand triangle opposite "Catalog info," but when clicked, that only displays more detail about the source PSE catalog I have already upgraded. There doesn't seem to be any way to browse to or select a second PSE catalog — which is what I'm trying to do.