The easiest way to convert an iPhoto library to Photos is with the trusted 3rd party app PowerPhotos ($35,) but that does cost some change. You can see how the conversion goes at their site, here:
Converting iPhoto and Aperture libraries
As the above says, even without buying PowerPhotos, you can use Photos' menu "File>Import" to import the pictures from a iPhoto library, but you loose all the organization and much of the detailed edits and metadata. The value of PowerPhotos is that it preserves all that information, and you can even load the iPhoto library into PowerPhotos beforehand and it lets you look through it before you do anything. I think the free version of PowerPhotos lets you have a look at how the results would go.
I'm not sure if PowerPhotos tests pictures and videos to see if they are compatible before importing. Some files, especially early ones before Photos went to 64 bit data, may foul things up. For such reasons, it's probably best to create a fresh Photos Library from the iPhoto library and make sure it works. PowerPhotos leaves the iPhoto library intact.
So, you can import bare unmodified pictures directly to Photos, or you can preserve structure edits and comments with PowerPhotos.
What do you think?