If you have been using iPhoto Library Manager to keep track of your iPhoto libraries, PowerPhotos can help you with your move to using the new Photos app.
Migrating your iPhoto libraries
In order to see your libraries in PowerPhotos, you will first need to migrate your libraries from iPhoto to Photos. PowerPhotos can assist with this process if you select “Migrate iPhoto Libraries” from the File menu. The migration assistant will list all the iPhoto libraries on your machine and let you migrate any number of libraries to Photos in succession. You can read in more detail about how this works at Migrating iPhoto and Aperture libraries.
If you have already migrated some or all of your libraries before downloading PowerPhotos, you can also select “Add Existing Library” from the File menu to add any of your existing Photos libraries to the PowerPhotos library list.
Merging, rebuilding, or copying photos between your libraries
Just as iPhoto Library Manager can copy and merge photos between iPhoto libraries, PowerPhotos can be used to copy/merge with Photos libraries. Due to limitations in the new Photos app, there are some pieces of data that iPLM could copy that PowerPhotos cannot. You can read more about that in Copying limitations. There is no way to copy photos directly between an iPhoto library and a Photos library. You must first take the iPhoto library and migrate it to Photos, then copy the photos from the migrated library to your other Photos library.
If you have an existing license for iPhoto Library Manager, you can upgrade to PowerPhotos 2.0 for a discounted price. Read more about registration and upgrading.