How do I open old iPhoto library

Photos can open iPhoto Libraries, that have been created with iPhoto '11 or iPhoto '9.

Very old iPhoto Libraries, that have ner been upgraded to the format of iPhoto '11 or iPhoto '9, need to be prepared with the iPhoto Library Upgrader tool: http://support.apple.com/kb/HT5037?viewlocale=en_US&locale=en_US This tool is a very old 32-bit application can only run on macOS 10.14 Mojave or older. We can no longer use it, if we already upgraded to macOS 10.15 Catalina or later.

If you already opened an iPhoto Library in Photos, the Photos.app will have changed the filename extension of the iPhoto Library from .photolibrary to .migratedphotolibrary

Change the filename extension back to .photolibrary , then Photos will open the library again, when you launch Photos while holding down the options key ⌥, until the library chooser dialog appears. Select the renamed iPhoto Library.

Photos will create a new Photos Library from the iPhoto Library, on the same volume, where the iPhoto Library is located. You have to ensure, that the iPhoto Library is located on a volume, compatible with Photos, as described here: Move your Photos library to save space on your Mac - Apple Support