Combining IPhoto & Photo Libraries

The first thing to do is launch Photos with the option key held down. From the dialog, choose your iPhoto Library. Photos will create a new library with your iPhoto Media.

Use Photos to make sure that your images are there, and double click on a sample to make sure you can edit them.

Next get FatCat softwares (paid) PowerPhotos. This will let you combine libraries and perform other maintenance.

https://www.fatcatsoftware.com

I am not sure where you are seeing a "numeric identifier" in iPhoto, I have never used it.

Many folks do prefer Events to the way Photos does it, but it is like admiring tail fins on Cadillacs, they are simply not coming back. Feel free to send feedback to Apple*, but Photos is what it is.

*Product Feedback - Apple

I read your response on moving iPhoto to Photo. And I thank you. My consternation is I have already moved many events into albums in Photo. However, Photo doesn't appear to have near the functionality of iPhoto. The two items I really miss are:

1) The numerical identifier assigned to each photo in iPhoto and is readily viewed when looking at photos. This is one of its most important features. Is there a way this identifier can be turned on? 2) by scrolling over the event, the contents are quickly revealed.

In spite of many peoples support of Photo, I can't stand its organizational structure. For me creating events is far more efficient and useful for the way I do business, and is the reason I haven't upgraded my Mac since Catalina

The numerical identifier

Or file name, which is assigned by your camera. Photos defaults to showing the Title rather than the filename. There ae scripts about that will write the filename to the title slot.

Events, as a concept, don't exist in Photos.

If Photos does not meet your needs then use a different app. It's not compulsory in any way and there are plenty of alternatives. What is clear is that iPhoto is dead almost seven years now, and it's not coming back.

What computer and operating do you have that has the Photos Library?

I am not sure where you are seeing a "numeric identifier" in iPhoto, I have never used it.

I guessed this was the filename: IMG_234.jpg etc

Of course, that comes from the camera, not iPhoto, and the same is in Photos.