Transfer from iPhoto to Photos
Is it possible to transfer photos from iPhoto to photos while leaving them in iphoto also?
Sure. Launch Photos with the option key held down. From the resulting dialog, choose your iPhoto Library. Photos will create a new library from your iPhoto media. your iPhoto Library will not be changed.
Note too, that Photos will use hard links to refer to the originals in the iPhoto Library, so you will save on disk space.