Warning: Users still using iPhoto considering upgrading to Catalina
If you're still using iPhoto 9.6.1 and are considering upgrading to Catalina I suggest you do the following:
1 - run iPhoto First Aid on the library: Repair Library Permissions, Rebuild Thumbnails and Rebuild Library Database. This is to give Photos the cleanest iPhoto library to migrate.
If you're on Mojave or High Sierra where you can migrate to Photos do so and check to see if all of your albums, projects and photos are migrated correctly. If you have problems you can open the iPhoto library with iPhoto and check its integrity and try migrating again.
If you're on an earlier system, Yosemite, El Capitan or Sierra, they had versions of Photos which still had bugs. You can try migrating and checking to see if the migrations went OK. Then when you upgrade to Catalina you can upgrade the Photos library and if it isn't to your liking try migrating your iPhoto library.
Hopefully this pre-upgrade preparation will provide you with a smooth and accurate iPhoto to Photos migration.
Here are some user tips from Photos for Mac User Tips that will help you in your transition from iPhoto to Photos:
Notes on Migrating an iPhoto Library from iPhoto to Photos for Mac
Moments in Photos are the New Events.
External Editors in Photos Are Here
Photo Editing Extensions for Photos for Mac
Post 9/30/2018 Printing Options for Photos - Apple Community
Ordering Books and Calendars on a Mac that cannot be updated past El Capitan
Use Project Extensions to order print products in Photos after September 30, 2018 - Apple Support
Create and order print products in Photos with Project Extensions - Apple Support
Good luck with your migration and upgrade.