To familiarize yourself with the new environment, you could first install PictureGuide alongside Picasa 3.
However, PictureGuide works independently of Picasa, so that the software can be uninstalled later.

Add all the folders you want in PictureGuide. (Comparison with Picasa - File - Add Folder..)
Via Settings - Library - Add Folder.

Unlike Picasa, PictureGuide checks all added folders for changes every time you start the program. Recently used folders are automatically monitored live. This does not have to be set separately.

PictureGuide now reads all images and automatically takes over the person labels from Picasa.
Unfortunately, information outside of the names (e.g. albums) cannot be transferred from Picasa.
If the images contain GPS coordinates, however, the location information will be added again immediately.

Subsequent changes to the person labels in Picasa are only applied to PictureGuide if the name tags are stored in photos in Picasa.