How to convert iPhoto to Photos in Monterey
How do I merge these old photos in iPhoto to Photos without losing any pictures
iMac 21.5″, macOS 10.13
First you migrate the iPhoto Library to Photos. This you do as always: open the iPhoto Library with Photos: Hold down the option (or alt) key and launch Photos. From the resulting menu select 'Choose Library' and point it at the iPhoto Library. It will do the rest.
If you then want to merge this library into an existing Photos library you can do this with Photos v7, if you have it. Or, if not you can use PowerPhotos: