Is there any way of retrieving the iPhoto pictures and transferring them to photos?

As you know Photos has replaced iPhoto.

Launch Photos with the option key held down, from the dialog, choose your iPhoto Library. Photos will make a new Library from your iPhoto media. Your iPhoto Library will not be changed, though it will be renamed.

The library upgrader tool is only used if you were running a Photos version earlier than about 9.

See Keith's reply, above?

Launch Photos with the option key held down, from the dialog, choose your iPhoto Library. Photos will make a new Library from your iPhoto media. Your iPhoto Library will not be changed, though it will be renamed.

Yes. Use the iPhone Upgrader Tool: https://support.apple.com/en-us/HT202358

Tge "Photo upgrader tool" has no role in this process.

To transfer iPhotos to Photos

How to